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A TEXTBOOK OF BOTANY FOR COLLEGES 



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THE MACMILLAN COMPANY 

NEW YORK • BOSTON • CHICAGO • DALLAS 
ATLANTA • SAN FRANCISCO 

MACMILLAN & CO., Limited 

LONDON • BOMBAY • CALCUTTA 
MELBOURNE 

THE MACMILLAN CO. OF CANADA, Ltd. 

TORONTO 



A TEXTBOOK OF BOTANY 



FOR COLLEGES 



BY 



WILLIAM F. GANONG, Ph.D. 

PROFESSOR OF BOTANY IN SMITH COLLEGE 



'Nzta gork 
THE MACMILLAN COMPANY 

1916 

All rights reserved 



Copyright, 1916, 
By the MACMILLAN COMPANY. 



Set up and electrotyped. Published August, 1916. 



AUG 31 1916 



Nnrbjonli iPress 

J. 8. Cashing Co. — Berwick & Smith Co. 

Norwood, Mass., U.S.A. 



)aA438193 



PREFACE 

This book is written in the knowledge that to nearly all 
college students an introductory course in Botany is part of 
a general education, and not a preparation for a professional 
botanical career. The distinction is important because our 
existent courses are largely adapted, albeit unconsciously on 
our part, to the latter end. The needs in the two cases are 
not the same, though the difference is less in matter and 
method than in proportion and emphasis. All students alike 
need that personal contact with specific realities, and that ex- 
ercise in verifiable reasoning, which laboratory courses render 
possible. Knowledge, however, is valuable to the specialist 
in the proportions of its objective importance, but to the gen- 
eral student in the proportions of its bearing on the actions 
and thoughts of mankind. In the one case the demands of 
the science are paramount and in the other the interests of the 
student. 

In conformity with its aim, the book gives more attention 
to the large and visible aspects of plant nature than to the 
minute and obscure. To the general student the things he 
can see in the world, and will meet with again, are more im- 
portant than those which lie remote from his path, though the 
specialist must know both near and remote, because both exist. 
Especially the book lays great emphasis upon interpreta- 
tion, or the explanation of the " principle " of things, and the 
connections of botanical science with the general body of 
knowledge, and man's direct relations with plants. Indeed 
the book may be described as an attempt to present and inter- 
pret the humanly important aspects of plant nature in the 



VI PREFACE 

light of our modern scientific knowledge. While these are 
not the matters the specialist needs most to know, I cannot 
but think that he also will find advantage in entering upon 
his work through this broader portal 

The book is supposed to be used in conjunction with organ- 
ized laboratory work, and to be read for the sake of connecting 
the discontinuous though invaluable knowledge won by expe- 
rience in the laboratory with the systematized content of the 
science, the two being welded thus into one intellectual unit. 
This assumption of contemporaneous laboratory work, sup- 
posed always to precede the reading, will explain a much 
greater generality or abstractness of treatment than would 
otherwise be suitable. Since, however, teachers differ much 
in their ideas as to desirable sequence and emphasis, I have 
treated the various topics in the form of semi-independent 
essays, intended to be separately understandable. The method 
involves repetition, but permits omission, by sections, where 
the material is found overabundant, as it will be for most 
students, though it should not prove so for the best. 

The fact that the book is prepared for the general student, 
whose psychology I have long been studying (when I might 
have been better employed, as I know my investigating col- 
leagues think), will explain some features not otherwise obvi- 
ous. Thus, structure is treated before function, because that 
is the more practicable way, even though the reverse is more 
logical. Again when the seemingly obvious is elaborated, it 
is because experience has shown how different is the aspect 
of those matters to the youthful beginner and the mature 
specialist. Further, if not all of the newest matters are in- 
cluded, it is not necessarily because I do not realize their 
scientific importance, but because, in most cases, they seem 
either not sufficiently established or not sufficiently prominent 
for inclusion in an introductory course. The test of the value 
of the book will be found not in whether my colleagues con- 
sider it a well-proportioned compendium of botanical fact, but 
in whether it leads students to pursue the subject in an inter- 
ested and spontaneous spirit. 



PREFACE Vll 

The illustrations are taken from many sources, the best I 
could find. I deem it. as legitimate to use a good published 
picture as a good published idea, of course with due credit ; 
and, moreover, its use seems such a deserved tribute to its 
excellence as its author would desire. Many are taken from 
the well-known works of Sachs, G-oebel, Kerner, and Stras- 
burger, and are so good that none better can be made ; and we 
should not deprive the student of their use, or waste the labor 
of providing inferior new ones, only because through frequent 
repetition they have become wearisome to us. Kerner's work 
is issued in translation in this country by Messrs. Henry Holt 
& Company, and this firm has given me full permission to use 
these pictures, as well as two from Sargent's Plants and their 
Uses, and several from my own book The Living Plant, pub- 
lished by them. Also I have used many, by permission, from 
publications of The Macmillan Company, and especially from 
one of the greatest of botanical publications, the Cyclopedia 
of American Horticulture, edited by Professor L. H. Bailey, 
who has graciously granted me the privilege of drawing at 
will from that work. The Bausch & Lomb Optical Company 
have kindly loaned me several cuts from their catalogues illus- 
trating apparatus of my own invention made by them. The 
new illustrations, comprising about a third of those in the 
book, have been mostly drawn by my colleagues in the depart- 
ment of Botany at Smith College, — three by Professor Julia 
W. Snow, two of the most elaborate by Professor Grace Smith, 
several by Miss Helen Choate, and many by Miss Marion 
Pleasants. A few of the diagrammatic figures are my own. 
These skilled co-workers, with another. Miss Grace Clapp, 
have also given me the advantage of their expert knowledge 
in a critical reading of the proofs. I am under special obliga- 
tion, however, to Miss Choate and Miss Pleasants, who, not only 
through their drawings, but also through their constructive 
criticisms, have contributed greatly to the merit of the book, 
though I claim its faults as wholly my own. To all of these 
generous collaborators I express my grateful acknowledg- 
ment. 



Vlii PREFACE 

Part II, containing the description of the groups of plants, 
comprising about 125 pages, is delayed, but is expected to be 
ready within a year. It will be issued separately for a time, 
but the two parts will also be bound in one volume. 

W. F. GANONG. 

June 20, 1916. 



CONTENTS 



INTRODUCTION 

Chapter L The Scope and Value of Botanical Study . 
Chapter II. The Distinctive Characteristics of Plants 



PAGE 
1 



PART I 



THE STRUCTURES AND FUNCTIONS OF PLANTS 

Chapter III. The Morphology and Physiology of Leaves . 

§ 1. The distinctive characteristics of leaves . 

§ 2. The structure of leaves .... 

§ 3. The synthesis of food by light in leaves . 

§ 4. The cellular anatomy of leaves . 

§ 5. The characteristics of protoplasm 

§ 6. The water-loss, or transpiration, from plants 

• § 7. The adjustments of green tissues to light . 

§ 8. The various forms of foliage leaves . 

§ 9. The forms and functions of leaves other than foliage 

§ 10. The nutrition of plants which lack chlorophyll 

§ 11. The autumnal and other coloration of leaves . 

§ 12. The economics, and treatment in cultivation, of leaves 

§ 13. The uses of the photosynthetic food .... 

Chapter IV. The Morphology and Physiology of Stems 

§ 1. The distinctive characteristics of stems 

§ 2. The structure of stems and support of the foliage 

§ 3. The cellular anatomy of stems .... 

§ 4. The development of stems and leaves from buds 

§ 5. The arrangements of leaves on stems 

§ 6. The transfer of water and food through plants 

§ 7. The growth of stems and other plant parts 



15 
15 
17 
19 
28 
35 
43 
52 
58 
72 
82 
88 
94 
97 

113 

113 
115 
128 
135 
139 
144 
153 



CONTENTS 



§ 8. The respiration of plants 162 

§ 9. The geotropism of stems 174 

§ 10. The various forms of foliage-bearing stems . . .179 

§ 11. The forms and functions of stems not connected with 

support of foliage 191 

§ 12. The monstrosities of stems and leaves .... 196 

§ 13. The economics, and treatment in cultivation, of stems . 205 



Chapter V. The Morphology and Physiology of Roots . 212 

§ 1. The distinctive features of roots 212 

§ 2. The structure of roots 215 

§ 3. The cellular anatomy of roots . . . ... 220 

§ 4. The absorption of vv^ater, and other functions of roots . 224 

§ 5. Osmotic processes in plants 232 

§ 6. The composition and structure of soils .... 237 

§ 7. The self-adjustments of roots to prevailing conditions . 247 

§ 8. The additional, and substitute, functions of roots . . 250 

§ 9. The economics, and treatment in cultivation, of roots . 257 

§ 10. Summary of the functions and tissues of plants . .261 



Chapter VI. The Morphology and Physiology of Flowers 

§ 1. The distinctive features of flowers . 

§ 2. The structure of flowers 

§ 3. The accomplishment of fertilization by flowers 

§ 4. The nature and consequences of fertilization . 

§ 5. The methods and meaning of cross-pollination 

§ 6. Methods of asexual reproduction 

§ 7. The origin and significance of sex . 

§ 8. Heredity, variation, and evolution . 

§ 9. The methods used by man in breeding better plants 

§ 10. The morphology of flowers .... 

§ 11. The morphology and ecology of flower clusters 

§ 12. Special forms, abnormalities, and monstrosities of flowers 

§ 13. The economics, and treatment in cultivation, of flowers . 



Chapter VII. The Morphology and Physiology of Fruits 

§ 1. The distinctive characteristics of fruits 

§ 2. The structure and morphology of fruits . 

§ 3. The dissemination and dispersal of plants 

§ 4. Special forms and monstrosities of fruits . 

§ 5. The nature and cure of plant diseases 

§ 6. The economics and cultivation of fruits . 



267 
267 
269 
276 
279 
286 
298 
302 
308 
317 
322 
335 
340 
343 

345 
345 
347 
356 
366 
367 
370 



CONTENTS 



XI 



Chapter VIII. The Morphology and Physiology of Seeds 
§ 1. The distinctive characteristics of seeds 
§ 2. The structure, morphology, and functions of seeds . 
§ 3, The suspension of vitality, resting period, and duration 

of life in seeds 

§ 4. The germination of seeds 

§ 5. The economics and cultivation of seeds . 
§ 6. The cycle of development from seed to seed 



PAGE 

372 
372 
373 

377 
381 
385 



PART II 
THE KINDS AND RELATIONSHIPS OF PLANTS 



/ 



A TEXTBOOK OF BOTANY 
FOR COLLEGES 

INTRODUCTION 

CHAPTER I 

THE SCOPE AND VALUE OF BOTANICAL STUDY 

The word Botany came originally from the Greek, where 
it meant simply grass, or herbage, especially that of a pasture. 
Its meaning, however, has expanded step by step with the 
progress of knowledge, until now it embraces every kind of 
scientific inquiry about plants. Thus the scope of the word, 
as of the science, has indeed become great. In the first 
place, plants themselves are wonderfully diverse in appear- 
ance, structure, and habits, for they comprise not only the 
familiar trees, shrubs, and herbs, with ferns, mosses, and sea- 
weeds, but also the mushrooms, molds, yeasts, and germs of 
disease and decay. Furthermore, the number of distinct 
kinds, or species, is far greater than most people imagine. 
Of plants having flowers, no less than some 133,000 separate 
species have already been described and named by botan- 
ists, while of the flowerless kinds, which reproduce by spores, 
some 100,000 species are likewise known, making 233,000 in 
all. It is believed, however, that a good many others re- 
main to be discovered, probably enough to bring up the 
number of the flowering kinds to 150,000 and of the flower- 
less to the same number, making at least 300,000 in all. As 
to the kinds of facts which botanists are trying to discover 
concerning this multitude of diversified plants, there are 
no limitations, because no bounds exist to the intellectual 



2 A TEXTBOOK OF BOTANY [Ch. I 

curiosity of scientific men, nor is there any way of deter- 
mining in advance which new facts will prove interesting to 
them or important to mankind. 

The study of Botany is pursued for three purposes, — 
pleasure, progress, and profit. First, as to pleasure, its 
pursuit in any intellectual field is one of the most rational 
and elevating of human activities. There are those who 
take as much delight in a close personal acquaintance with 
plants, or in a clear understanding of their construction and 
processes, as others find in a knowledge of literature, history, 
art, or the drama; and the one pursuit is entitled to the 
same s^onpathetic approbation as the others. Second , as 
to progress, all experience shows that an individual advances 
precisely as a race does, — through constant intellectual ef- 
fort ; and for such exercise there exists no more natural field 
than the scientific investigation of the surrounding world, of 
which plants comprise the most conspicuous part. Third, as 
to profit, that is clear when one recalls the intimacy of man's 
dependence upon plants for the very essentials of civilized 
existence, — for food, shelter, raiment, and medicine, — in 
conjunction with the fact that they are readily capable of 
improvement under his hand, as attested by the magnificent 
flowers, luscious fruits, and nutritious vegetables which he 
has developed from insignificant wild ancestors. The fact 
that man can make plants serve still better his material uses 
would be reason enough, even were there no others, why 
he should study them thoroughly. 

Thus the science of Botany has a scope far too vast, and 
a body of knowledge much too great, for any one mind to 
grasp. Therefore it has become subdivided for purposes of 
exact investigation. From this point of view, all Botany falls 
into four divisions, and they into subdivisions, as follows. 

I. Systematic Botany, the oldest and most fundamental 
of the divisions, now commonly called Taxonomy, is con- 
cerned chiefly with the Classification of plants, that is, 
their arrangement in groups in accordance with their relation- 



Ch. I] THE VALUE OF BOTANICAL STUDY 3 

ships to one another. It includes exact description of the 
species, and apphcation of scientific names, which are taken 
from Latin, as the principal language of learning. It has 
been studied mostly by observation and comparison of the 
prominent external parts of plants, especially the flowers 
and fruits ; and for the convenience of such study, the plants 
are preserved in a pressed and dried condition in collections 
each called an Herbarium. For the use of students and 
other workers with plants, the classification, descriptions, and 
names of all the plants of a country are embodied synop- 
tically in handbooks, commonly called Manuals (or, if 
more elaborate. Floras), so arranged as to enable a student 
to find for himself the correct name of a plant previously 
unknown to him. An important subdivision of Systematic 
Botany is Paleobotany, or the study of the plants which 
existed in past ages, as represented in their petrified, or fossil, 
remains found in the rocks, — a subject which throws great 
Hght upon the evolution of our present plants from their 
remote and very different ancestors. 

II. Morphology, second in age of the divisions, is the 
study of the parts, or structures, of plants, in comparison 
with one another. It therefore bears much the same rela- 
tion to the parts of plants that classification bears to plants 
as a whole ; and it is studied by the same methods of ob- 
servation and comparison. When it leads from the large 
external to the small internal parts, thus requiring the aid 
of the microscope, it takes the name Anatomy, while if it 
goes deeper yet, into the minute construction of the ulti- 
mate smallest parts (called cells), it is termed Cytology, — 
the two latter terms together replacing the older term His- 
tology. An important phase is Embryology, the study of 
the stages in development of the individual before its 
birth or germination, all of its stages collectively constituting 
its "life-history." 

HI. Physiology, third in age of the divisions, is precisely 
the same study in connection with plants as it is with ani- 



4 A TEXTBOOK OF BOTANY [Ch. I 

mals, including mankind, viz., the study of the organic pro- 
cesses or functions. It is pursued by the exact experimental 
methods of physics and chemistry, and indeed may be de- 
scribed as the physics and chemistry of plant hfe. Dealing 
thus with matters of the most fundamental nature, its dis- 
coveries frequently prove not only of the highest scientific 
interest, but also, as will presently appear, of great economic 
importance. One of its phases, that which concerns the 
relations of structure and habit to the conditions under 
which plants live^ has attained to a prominence requiring a 
name of its own, viz.. Ecology, — a term which has largely 
absorbed the older word Plant-geography, meaning the 
distribution of plants in light of its causes. Still more re- 
cently another phase of physiology has become prominent, 
viz.. Genetics, the experimental study of the facts and 
methods of heredity. 

IV. Economic Botany, also known as Plant Industry, 
extremely old as an empirical study though very new as a 
scientific one, is the investigation of plants with reference 
to their improvement for the uses of mankind. It com- 
prises a number of well-known subdivisions, viz., scientific 
Agriculture, Horticulture, and Forestry, with others less 
famihar, viz.. Bacteriology, the study of disease germs, and 
other kinds ; Pharmacology, dealing with drugs ; Pathol- 
ogy (Phytopathology) concerned with the diseases of 
plants; and Plant-breeding, or the systematic attempt to 
produce new and superior kinds, — a subject closely inter- 
locked with Genetics. Economic Botany is the special 
field of Agricultural Experiment Stations maintained by 
civilized governments the world around, including the 
United States Department of Agriculture and the State Ex- 
periment Stations and Agricultural Colleges in this country, 
excepting that Bacteriology belongs primarily to the Medical 
Schools. The other three divisions, Systematic Botany, 
Morphology, and Physiology, are cultivated particularly in 
the Universities. 



Ch. I] THE VALUE OF BOTANICAL STUDY 5 

These divisions, and subdivisions, of Botany are pri- 
marily determined by convenience of study, especially with 
reference to the methods and instruments employed. Hav- 
ing really no natural boundaries, they intergrade and inter- 
lock very closely, on which account the progress of one 
depends upon progress of the others. Thus, most phases 
of Economic Botany are so dependent upon Physiology in 
particular, that the greater Experiment Stations, main- 
tained primarily for economic research, are well-nigh as 
active in Physiology as are the Universities. This case 
is typical of the relation which exists everywhere between 
economically useful and scientifically abstract knowledge. 
The history of civilization has shown that the greater ap- 
plications of science to human welfare, as exemplified in 
electricity, wireless telegraphy, or the control of germ diseases, 
have arisen not from researches directed to secure useful 
results, but incidentally as by-products of purely abstract 
investigations made in the pursuit of knowledge without 
thought of material returns. All experience shows that 
knowledge is a unit, of which economically useful knowledge 
is only an ill-defined and changing part ; and the surest way to 
gain new useful knowledge is first to win new general knowl- 
edge, which is possible only through scientific research. 
For this reason the student who aspires to become a leader 
in any economic pursuit must first make himself master 
of its general or abstract knowledge. Such is likewise the 
reason for the emphasis laid in education as a whole upon 
subjects having no apparent economic utihty. 

The facts known about plants being so multitudinous, 
amounting it must be to millions, and far beyond com- 
prehension by any one person, the student may well ask 
how it is possible to acquire that general understanding of 
plants implied in an introductory course, and textbook, of 
Botany. It is simply thus. The diversity of plants, so 
extensive and obvious, is really superficial, and rests upon 
foundations of similarity, which, deep, obscure, and dis- 



6 A TEXTBOOK OF BOTANY " [Ch. I 

coverable only by prolonged investigation, are relatively 
few in number. By utilizing these deep-lying resemblances, 
it is possible to link together great masses of facts in gen- 
eralized form, and thus bring the principles of botanical 
knowledge within the comprehension of one person, who 
may then pursue in detail any particular phase which his 
pleasure or business may dictate. 



CHAPTER II 
THE DISTINCTIVE CHARACTERISTICS OF PLANTS 

The Universe, wrote the great Linnaeus in the sonorous 
Latin of the "Systema Naturae/' comprises everything which 
can come to our knowledge through the senses. The Stars are 
very distant luminous bodies which circle in perpetual motion, 
and are either Fixed Stars shining by their own light like the 
Sun . . . or Planets deriving their light from the Fixed Stars. 
. . . The Earth is a planetary globe, rotating in twenty- 
four hours, moving in an orbit around the sun once a year 
. . . and covered by an imm-ense mantle of Natural Objects 
the exterior of which we try to know. . . . Natural Objects 
. . . are divided into three Kingdoms of Nature, Minerals, 
Plants, and Animals. . . . Plants are organized bodies 
which live but do not feel (or as we say, are not conscious). 

Such is the place in nature of plants, which the botanist 
is trying to know. 

Of these plants there are many distinct kinds or species, 
probably some three hundred thousand, as noted already. 
Each species, however, consists of thousands, or millions, 
or perhaps billions, of individual plants. 

Individual plants, of the familiar kinds, are each composed 
of six primary parts, — leaves, stems, roots, flowers, 
FRUITS, and seeds. Each part performs a particular pri- 
mary function to which it is fitted in structure. In the ex- 
panded thin green leaves food is made for the plant, under 
action of sunlight, from materials drawn from the air and 
the soil. The columnar elastic branching stems spread and 
support the leaves in the indispensable sunlight. The 
slender roots, radiating and ramifying through the soil, 

7 



8 A TEXTBOOK OF BOTANY [Ch. II 

absorb the water and mineral salts needed by the plant, to 
which they give also a firm anchorage in the ground. The 
showy and complicated flowers effect fertilization, which 
is requisite in all sexual reproduction. Fruits, whether 
dry like pods, or edible Hke berries, are concerned with the 
formation and dissemination of seeds. The compact hard- 
coated SEEDS, containing each an embryo plant and food 
supply, separating from the parent plant, and remaining for 
a time dormant, provide a transportable stage whereby 
plants are spread. Thus each of the six primary parts per- 
forms a definite function in the economy of the plant as a 
whole, and each part is therefore, from the physiological 
point of view, an organ. In addition each of these organs 
performs functions connected with its own individual ex- 
istence, notably growth, respiration, and self-adjust- 
ment to the surroundings. 

The external form of these primary parts, visible to the 
unaided eye, is correlated with a definite internal anatomy, 
revealed by thin sections viewed through magnifying lenses. 
Thus studied, the parts are found composed of definite and 
symmetrically arranged differentiations of structure called 
tissues, having each its distinctive position, color, and 
texture, and each performing a definite part of the organ's 
function. Thus the veins and green pulp are tissues of the 
leaf, as are bark, wood, and pith of the stem, though some 
of the latter are further divisible. These tissues in turn, 
when viewed by the compound microscope, are found wholly 
composed of very small structures called cells, which ap- 
pear as compartments separated by firm walls and holding 
various contents. Of these contents the most important is 
the PROTOPLASM, a mobile, gelatinous material, the seeming 
simphcity of which is behed by its many remarkable prop- 
erties. It is really the protoplasm which performs the 
functions of the plant, and which builds the cells, and there- 
fore the tissues and organs, suited in structure to the work 
which is done by the respective parts. 



Ch. II] CHARACTERISTICS OF PLANTS 9 

While typical plants all have the same organs, they are 
not all ahke, but differ greatly in habits, aspect, and details 
of structure. Some are trees, tall, long-hved, and single- 
trunked, forming the canopy of forests. Others are shrubs, 
shorter and less lasting, branching from the ground, and 
forming the typical undergrowth. Others are herbs, smallest 
and shortest-lived of all, soft-bodied and mostly green 
throughout, forming the carpet vegetation of the earth. 
Then there are plants which grow supported upon others, 
the CLIMBERS and epiphytes : and the plants of strange 
aspect found in the deserts: and the water-plants, in- 
cluding the seaweeds : and all of the great number of the 
small and simple parasites, which occur everywhere amongst 
other plants. Some kinds possess organs other than those 
we have mentioned, such as tendrils, pitchers, and 
TUBERS, always associated with special habits ; but these parts 
prove on comparative study to be mostly transformed leaves, 
stems, or roots, though not all special structures have this origin. 

The organs develop in the individual plants in definite 
predetermined cycles. Every plant normally originates in 
a fertihzed egg cell, as does the animal in an egg. The 
egg cell, lying within the ovule inside the flower, is a 
microscopic protoplasmic sphere, at first without organs; 
but in the course of development it forms a stem and a few 
leaves, in which stage it is an embryo within a seed inside a 
fruit. When, after dissemination, the seed germinates, the 
embryo develops a root, and more stem and leaves, becom- 
ing a seedling, and with further repetition of those parts, 
ultimately an adult plant. Then it begins reproduction 
by developing flowers, in which sexual cells, egg cells 
and SPERM CELLS, are formed and brought together, making 
new fertilized egg cells, thus closing the cycle, which is re- 
peated in perfect regularity, generation after generation. 

Plants are not, however, merely aggregates of parts per- 
forming present functions, but include many rehcs of their 



10 A TEXTBOOK OF BOTANY [Ch. II 

lives in the past. The evidence seems to show beyond 
question that our present species of plants have descended 
by gradual evolution from simpler and fewer species which 
formerly existed, and which in turn were evolved from still 
simpler and fewer kinds, — back, it is possible, to a single 
kind which throve in remotest antiquity. In the course of 
this evolution, plants have diverged into the many groups, 
and groups within groups, expressed in our schemes of classi- 
fication. Thus also various features originally distinctive 
of one species came to prevail through whole families, and 
even persist to the present, often having lost completely 
their original significance. It is the aim of botanists to 
distinguish between those features which have merely a tem- 
porary functional significance and those which are deeply 
fixed in heredity. They use the former in the interpreta- 
tion of the phenomena of plant life, and the latter as guides 
to evolution and classification. Hence botanical study falls 
most fundamentally into the two phases represented by the 
two Parts of this book. 

While the groups and classification of plants will receive 
full treatment in Part II, some general knowledge of the more 
important of such facts is essential to an understanding of 
Part I. The main groups, with their essential character- 
istics, are the following. 

1. The Flowering Plants, the most highly evolved 
and therefore often called the "higher plants," comprise the 
great majority of the trees, shrubs, and herbs constituting 
the famihar land vegetation. They are distinguished not 
only by the possession of flowers, which often are extremely 
inconspicuous, but also, and especially, by their seeds, on 
which account they are called sc'entifically Spermatophytes, 
that is, ''seed plants." While mostly they dwell on the land 
with roots in the ground, and make their food in their 
green leaves, some live in water, and some upon other 
plants. They are clearly descended from the following 



Ch. II] CHARACTERISTICS OF PLANTS 11 

group, which is much older, as shown by fossil remains in 
the rocks. 

2. The Ferns and their kin, called scientifically Pteri- 
DOPHYTES or ^'Fern plants," comprise not only the familiar 
true Ferns, but also the less prominent Horsetails and Club 
Mosses. They have no flowers, but reproduce by small one- 
celled spores and a definite though not prominent sexual 
stage. They live chiefly on land, have green leaves, and 
make their Own food. They are mostly undergrowth plants, 
though some in the tropics become trees. They have evolved 
(it is likely but not certain) from the following group, and 
were formerly more prominent than now, having once formed 
great forests, the earliest of such vegetation. 

3. The Mosses and their kin, called scientifically 
Bryophytes or ''Moss plants," comprise the true Mosses 
with the Liverworts. They reproduce like the Pteridophytes, 
by spores and a sexual stage. They have green leaves and 
make their own food, but they rise Httle from the ground, 
on which they grow densely together, thus forming the 
simplest carpet vegetation of the earth. They are de- 
scended from the Algae, and were probably the first plants 
to cover the land. 

4. The Molds and their kin, called scientifically 
Fungi, comprise a great number of small or minute plants 
most of which are found associated with the disease and 
decay of plants or of animals, e.g., mushrooms, yeasts, 
molds, rots, rusts, mildews, and bacteria, — popularly known 
as microbes or germs. They occur in the most diverse situa- 
tions, but always in contact either with living tissues, upon 
which they live parasitically, or else with dead organic 
substances, upon which they live saprophytic ally. They 
are most diverse in forms, sizes, colors, and other features, 
in accordance with their particular habits, but never show 
the green color of the higher plants. They reproduce by 
minute spores, which are carried everywhere by the winds, 
thus explaining how those plants can occur in so many 



12 A TEXTBOOK OF BOTANY [Ch. II 

situations. They are undoubtedly descended, as shown by 
many resemblances in structure, from the Algae ; and so close 
are their relationships that, from the point of view of classi- 
fication, the two groups are properly included in one, called 
Thallophytes, though in practice it is convenient to treat 
them separately. 

5. The Seaweeds and their kin, called scientifically 
Alg^, comprise not only the red and brown seaweeds and 
"sea mosses" (which are green underneath those colors), 
but likewise many green kinds both of salt and fresh water. 
They live mostly under water, make their own food in their 
fronds, have diverse shapes with different habits, and re- 
produce both by simple spores and sexual stages. They are 
the simplest and most ancient of the leading groups, and 
the one from which the others are descended. 

Algse, Fungi, Bryophytes, and Pteridophytes are often 
called collectively Cryptogams, because their reproduction 
was once thought obscure, while the Spermatophytes are 
called Phanerogams, because their reproduction, through 
flowers, was considered evident. 

It is the primary aim of science to discover, analyze, de- 
scribe, and classify the elemental facts of nature. It is a 
secondary aim to explain phenomena with which the facts are 
connected, though to all except specialists the explanations 
are hardly inferior in interest to the facts themselves. In 
this book, while the description of fact always comes first, 
explanations follow promptly after. The explanations of 
the phenomena exhibited by living plants fall under four 
categories. First, a great many features, especially those 
connected with the obvious fitness of form and structure 
to functions and habits, are best explained, in the opinion 
of a majority of biologists, as result of a process of gradual 
ADAPTATION of the modifiable plant to the unmodifiable 
physical surroundings during the course of evolution. Second, 
other features are clearly survivals, of no other present sig- 



Ch. II] CHARACTERISTICS OF PLANTS 13 

nificance, from ancestral forms, as noted already under 
HEREDITY. Third, plants are still in process of evolution, 
and hence, for causes and by methods still unknown, are con- 
stantly developing new features called variations, or better, 
— MUTATIONS. Fourth, the adaptations, the heredity, and 
the mutations of plants are all more or less affected, and 
even in some degree directed, by the chemical nature of the 
materials they are composed of, and the physical forces playing 
upon them from the world in which they live ; and on this 
account many of their features have a purely incidental, or 
mechanical, or, as we may designate them collectively, struc- 
tural significance. Thus the actual plant embodies the 
resultant of the simultaneous action upon it of adapta- 
tional, hereditary, mutational, and structural, with some 
other minor, factors. It is the task of the botanist to dis- 
tinguish and separate the various influences which make the 
plant what it is, for which purpose he needs above all an 
open mind, a wilhngness to weigh all forms of evidence, and 
freedom from the human but unscientific tendency to adopt 
some single favorite viewpoint and explain all phenomena 
therefrom. Many matters in science are interpreted dif- 
ferently by equally competent investigators, but discussion 
and further investigation always bring the truth, for the 
recognition of which we have only one test, — it is that 
upon which the great majority of competent investigators, 
after full and disinterested investigation, agree. 

The generalized statements of this chapter are intended 
to enable the student to approach his study with better 
understanding. We turn now to the concrete facts and 
phenomena of plant nature. 



PART I 

CHAPTER III 

THE MORPHOLOGY AND PHYSIOLOGY OF LEAVES 

1. The Distinctive Chakacteristics of Leaves 

Leaves are the most abundant and conspicuous of plant 
parts, collectively constituting foliage, the most distinctive 
part of vegetation. Their essential features consist in their 
green color, flat form, and growth towards light. Their 
prominence is explained by their function, which consists 
in the exposure of green tissue to light, under action of which 
the plant forms its food out of water and mineral matters 
drawn from the soil, and a gas received from the air. This 
function is all the more important because the food thus 
formed serves not only for plants, but ultimately for all 
animals as well. 

Although uniform in their .primary function, foliage 
leaves show much diversity in various features. In size, 
some are almost microscopic, most are a few square inches 
in area, and a few are measured in feet. In shape, some are 
nearly circular, others almost needle-form, and others of 
diverse intermediate gradations. In color, while typically 
green, some are gray, white, yellow, or red ; and in autumn 
they often display a brilliant succession of colors. In tex- 
ture, some are flaccid, as in water plants, others almost 
leathery, as in evergreen trees, while most are intermediate, 
with a flexible-elastic consistency. In duration of life, they 
are typically temporary, lasting but one season, and even in 
evergreens for only a few years ; but cases occur in which the 
leaves persist as long as the long-lived stem. In only one 

15 



16 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 1 



feature do foliage leaves vary little and that is the thickness, 
or rather the thinness, of their green tissue, which is nearly 
the same no matter what their sizes and shapes. 

The thin flat expanse of green tissue, called the blade, 
is always the essential, and often the only, part of the leaf. 
In many kinds, however, the blade is provided with a slender, 
cylindrical stalk, called the petiole, various in length even 

up to several feet ; and upon it the 
blade is adjusted to the light, and 
has free play in the wind. In addi- 
tion, some kinds possess a pair of 
small appendages, one on each side 
of the base of the petiole, called 
STIPULES, which, though usually 
green like the blade, are very diverse 
in form. Blade, petiole, and stipules 
are parts of a complete leaf, of which 
a typical example is pictured here- 
with (Fig. 1). 

In some kinds of leaves, es- 
pecially large ones, the blade is 
not all one piece, but is cleft more 
or less into divisions, as familiar in 
Oak or Maple. The same process 
continued much farther results in 
the formation of separate leaflets, 
each with a stalk of its own, as in Rose or Strawberry 
(Fig. 37), while the leaflets also may become themselves 
subdivided, even more than once, as in some kinds of Ferns. 
Such leaves are called compound, in distinction from simple, 
the two being distinguishable by the fact that the leaflets 
of a compound leaf always stand in one flat plane, while 
j^mple leaves are distributed around a stem, at least at their 
bases. Further, leaflets have no buds in their axils, but 
leaves, whether simple or compound, always do. 

While typical leaves, the kinds designated foliage, are 




Fig. 1. — a leaf of the 
Quince, showing blade, petiole, 
and stipules ; reduced. (After 
Gray's Structural Botany.) 



Ch. Ill, 2] STRUCTURE OF LEAVES 17 

thin, flat, and green, and perform the function of food forma- 
tion, other kinds exhibit different features and other func- 
tions, as famihar, for instance, in tendrils and pitchers. 
Likewise there are parts which seem to be leaves but are not, 
as in case of some flattened stems, and even roots ; for leaves, 
while the principal, are not the only green parts of plants. 

2. The Structure of Leaves 

Typical, or foliage, leaves, despite their external multi- 
formity, possess an essentially uniform anatomical struc- 
ture, as shown by comparative observation. 

The most conspicuous and important part of the leaf, that 
in which the food is formed, is the green tissue, called chlo- 
renchyma, which is singularly uniform in thickness, texture, 
and color throughout the leaf blade. Its distinctive green 
color is not, however, an integral part of its structure, but 
a separate and easily removable substance. One has only 
to place a leaf in a glass dish, cover with alcohol, stand in 
a warm place, and leave for a time, when the green will 
come out in a beautiful clear solution, leaving the leaf a 
uniform white. This soluble green substance is called 
CHLOROPHYLL, and is one of the most important substances 
in nature, as will presently appear. 

Second in prominence is the system of veins, which 
ramify everywhere throughout the chlorenchyma. They 
are essentially bundles of tubes which conduct materials 
into and out of the chlorenchyma. Most commonly they 
taper and branch from the base of the blade towards the 
margin, simultaneously producing small veinlets which 
interlace to a network, as seen very clearly when held up 
against the light. In other kinds of leaves, such as Grasses, 
the main veins are uniform in size, and run parallel, or 
gently curving from base to tip, the veinlets in this case 
being minute or even wanting ; and such leaves are called 
PARALLEL-VEINED in distinction from the former, or netted- 
veined kinds (compare Figs, 1 and 2 with 34). If, further. 



18 



A TEXTBOOK OF BOTANY 



[Ci 



III, 2 



some typical leaf, e.g. from one of our common trees, be 
held up against the light and examined with a hand lens, 
one can see very clearly that the ultimate meshes of 

the network of vein- 
lets inclose little 
polygonal areas of 
pure chlorenchyma, 
into which often, 
though not always, 
extend free tips of 
the tiniest veinlets 
(Fig. 2). This ulti- 
mate relation of 
veinlets and chlor- 
enchyma is impor- 
tant, as will later 
appear. The small- 
est veinlets are 
buried within the 
leaf blade, but the 
larger ones and the 
veins which are pro- 
gressively thicker 
towards the leaf 
base, swell gradually 
out from the blade 
on its under side 
until they become 
many times thicker 
than the ever uni- 
form chlorenchyma. 
Third is the epi- 
dermis, a very thin 
and transparent 
layer by which all leaves are covered, and which often displays 
a shining surface when viewed obliquely towards the light. 




Fig. 2. — The vein systems of English Ivy 
(above) and Silver Poplar ; reduced. (From 
The Phantom Bouquet, by Edward Parrish, 1865.) 

The pictures were drawn from specimens 
"skeletonized" by removal of the chloren- 
chyma. A magnifying lens should be used to 
render visible the ultimate veinlets. 



Ch. Ill, 3] SYNTHESIS OF FOOD 19 

It is practically waterproof, and thus prevents desiccation 
of the soft leaf tissues when exposed to the sun and dry air. 
While tightly adherent, as a rule, to the chlorenchyma and 
veins, it can sometimes be stripped away, if started with a 
knife, from leaves of the Lily-like kinds, while from some 
of the Houseleeks (or ''Live for ever") it can be loosened 
by pressure of the fingers, and later blown out, as most 
children well know. Commonly the epidermis appears per- 
fectly continuous and homogeneous, but in exceptional 
cases {e.g. Wandering Jew), the hand lens will show, espe- 
cially on the under side of the leaves, tiny slit-like pores in- 
closed in greener ovals. These slits, called stomata, are always 
present, even though rarely visible to a hand lens. They are 
real openings, which connect with microscopical air passages 
extending everywhere through the leaf, and having great 
functional importance, as will soon appear. Also the epi- 
dermis, while typically smooth even to shining, often bears 
divers sorts of fine hairs or scales, called trichomes, which 
give to the leaves a grayish, woolly, or sometimes scurfy 
appearance whereby often the clear green of the underlying 
chlorenchyma is obscured. 

The petioles of leaves, typically cylindrical in form, consist 
mostly of veins, with little overlying chlorenchyma; but 
they develop commonly some additional strengthening tissue. 
The stipules, when present in typical form, have simply the 
leaf structure in miniature. 

3. The Synthesis of Food by Light in Leaves 

The prominence of leaves, in conjunction with their com- 
parative uniformity of structure, indicates for them a very 
fundamental function in plant life. This is well known to 
consist in the formation of food, which, as one of the most 
important of all processes in nature, will here be described 
somewhat fully. 

All leaves are found by chemical tests to contain sugar, 
mostly the kind called grape sugar, which occurs dissolved 



20 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 3 



in their sap. Under action of sunlight this sugar increases 
in quantity, but in darkness it lessens, because removed 
through the veins to the stem. Furthermore, in most leaves, 
when this sugar increases beyond a certain percentage the 
surplus becomes automatically transformed into starch, which 
returns again to grape sugar as the percentage thereof once 
more falls. Now it happens that starch (unlike sugar) is 
readily recognizable by a striking and easily apphed test, 
viz., addition of iodine in solution, which turns starch dark 
blue ; and thus we are provided with a convenient means of 

proving the increase 
of sugar, as manifest 
in its transformation 
to starch, under action 
of light. The experi- 
ment is well-nigh 
classic, and every 
student should see it. 
One has only to keep 
a thin-leaved potted 
plant for a day or 
two in the dark (to 
cause the disappear- 
ance of its starch) : 
cover part of a leaf, 
in a way not to pre- 
vent its ordinary func- 
tions, with some kind of contrasting hght-and-dark screen, 
such, for example, as shown in our picture (Fig. 3) : expose the 
plant to strong, but not intense, light for two or three hours : 
place the leaf in warm alcohol until the chlorophyll is re- 
moved : and cover the blanched leaf with a solution of iodine. 
Then a striking result appears, for the parts left in light 
by the screen all turn dark blue, and the parts which were 
shaded remain white, or at most a httle browned by the 
iodine (Fig. 4). Thus it is clear that the starch, and there- 




FiG. 3. — A light screen for experiment: 
in starch formation by leaves ; X h 

The star is cut from tinfoil attached to 
glass, and the box excludes light but admits air. 



Ch. Ill, 3] 



SYNTHESIS OF FOOD 



21 



fore the sugar, increases in quantity under action of light. 
Indeed so exactly quantitative is this relation of Hght to 
starch-formation that, with certain practical precautions, 
one may apply a photographic negative to a leaf, and after 
exposure to hght develop a very fair positive ''blue-print" 
of the picture with iodine. 

The increase of the grape sugar in hght is found by ex- 
periment to add weight to the 
plant. Therefore the sugar must 
represent not a transformation of 
material already present, but a new 
construction out of materials drawn 
from outside the plant; and all 
research confirms this conclusion. 
Further, suitable tests always show 
that its formation takes place only 
in Hght and only in green tissues, 
which never occur away from the 
light. Its production indeed is the 
particular primary function of the 
chlorenchyma, wherever found, 
whether in leaves, stems, or other 
parts, — the leaves being organs 
adapted to spread chlorenchyma to 
light. The formation of the sugar 
being thus a process of synthesis 
under action of light, is known as 

PHOTOSYNTHESIS. 

What now are the materials from which the grape sugar 
is constructed? 

The chemical formula of grape sugar is C6H12O6, which 
means of course that its molecule is composed of six 
atoms of carbon, twelve of hydrogen, and six of oxygen. 
Now the proportions H12O6 in this formula recall the 
familiar H2O, suggesting that water may be the source 
of that part of the sugar, at least of its hydrogen; and 




Fig. 4. — A leaf treated 
with iodine after exposure to 
light under the screen of Fig. 
3; Xi- The black shading 
represents dark blue in the 
actual leaf. 



22 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 3 



© 



this hypothesis is fully confirmed by research. The water 
is absorbed into the plant from the soil through the roots, 
conducted through the stem, and distributed through the 
veins to all parts of the chlorenchyma, from which its 
immediate evaporation is prevented by the waterproof 
epidermis. As to the carbon, that is known to come 
not from the soil (for plants can be grown to perfection 

in soils, or even in water, 
which lack it completely) , 
but from the air, in which 
it exists in the form of 
carbon dioxide (CO2), 
the heavy poisonous gas 
which is released by com- 
bustion and also by the 
respiration of animals. 
It is true, this gas is 
relatively scarce in the 
atmosphere, of which it 
comprises only about .03 
per cent (3 parts in 
10,000) as compared with 
about 21 per cent of 
oxygen, and 79 per cent 
of nitrogen; but even 
this small amount suffices 
for the photosynthetic needs of plants, as can be proved in 
various ways. Thus, one has only to keep a thin-leaved plant 
for a day or two in the dark to free it of starch : remove two 
similar leaves and place them in water in two glass chambers 
exactly alike except that from one all carbon dioxide has been 
removed by a chemical absorbent : expose them thus a few 
hours to fight : blanch them of chlorophyll : and immerse 
them in iodine, when there follows the result pictured here- 
with from an actual experiment (Fig. 5). Thus it is clear 
that a leaf can make starch, and therefore sugar, if the car- 





FiG. 5. — Leaves treated with iodine 
after exposure to light in air lacking and 
possessing, respectively, the usual carbon 
dioxide ; Xj- The black shading represents 
dark blue in the actual leaf. 



Ch. Ill, 3] 



SYNTHESIS OF FOOD 



23 



bon dioxide of the atmosphere is available, but otherwise not. 
Carbon dioxide cannot pass through the walls of the water- 
proof epidermis (at least not in appreciable quantity), but 
it enters the leaf through the slit-like openings, the stomata, 
the function of which is thus 
explained. From the stomata 
it moves along the air pas- 
sages to every part of the 
chlorenchyma. 

The formation of grape 
sugar from carbon dioxide 
and water is expressed by 
the following equation, which 
exhibits the extremes, though 
not the intermediate steps, of 
the process. 

6 CO2 + 6 H2O = CfiHisOe + 6 O2 

Now this equation implies 
that in the formation of the 
sugar, free oxygen is pro- 
duced in volume precisely 
equal to that of the carbon (seerin se^i"n)Therebnna^^^^^ 

proved that oxygen is released by 
green tissues in light ; X }• 

The gas released by the water 




Fig. 6. — A simple arrangement 



dioxide absorbed. This theo- 
retical deduction can readily 

be tested by experiment, by plant is caught in the water-filled 
_^ r >- r ' ± 1 test-tube supported above, and sub- 

means of apphances pictured ^equentiy tested. 
herewith (Figs. 6 and 7) ; and 

thus the actual production of oxygen, in the indicated vol- 
ume, is conclusively proved, and all parts of this photo- 
synthetic equation are found exactly true. It expresses 
concisely and accurately one of the greatest of all natural 
processes. 

The absorption of carbon dioxide and release of oxygen 
thus shown to occur in the photosynthetic formation of 
grape sugar in leaves explains the widely known fact that 



24 



A TEXTBOOK OF BOTANY 



[Ch. hi, 3 




plants (really only 
green plants in the 
light) ''purify the 
atmosphere," that is, 
remove from it the 
noxious carbon diox- 
ide released by ani- 
mals in their respira- 
tion (and by all com- 
bustion), and replace 
it by oxygen essential 
to animal respiration. 
Thus is a balance 
maintained between 
the two kingdoms. 
The oxygen released 
in photosynthesis 
represents merely an 
incidental by-product 
of the process. 

The amount of 
sugar made in a given 
time per unit area 
of leaf has been deter- 
mined for a number 
of plants, and shows, 
as would be expected, 
much diversity. The 
average of these fig- 
ures, however, ex- 



pressed in the nearest 



B & U 0. CO. 

Fig. 7. — A photosynthometer, by which the 

gas exchange in photosynthesis is quantitatively . 

tested ; X ^■ 

Into the chamber containing the leaves a known quantity of carbon dioxide is ad- 
mitted through the stop-cock from the graduated tube above. After exposure to light, 
analysis of the gas in the chamber is made by absorption in the graduated tube by aid of 
the two reagent tubes shown below on the left. The result can be read directly on the grad- 
uated tube, as shown on the left, where the approximate 28% indicates the oxygen present 
at the close of an experiment in which 10% of carbon dioxide had been added to the tube. 



I 



Ch. Ill, 3] SYNTHESIS OF FOOD 25 

round number, gives us a useful conventional expression, or 
constant, for the process as a whole, even though it has no 
validity as applied to any particular plant. This conven- 
tional CONSTANT for photosyuthesis, assuming the usual 
conditions of light, is 1 gram of grape sugar per square meter 
of leaf area per hour. This amounts to 10 grams per average 
working day, or 1500 grams per summer season, for that 
area. In the process 750 cubic centimeters of carbon dioxide 
are withdrawn from the atmosphere each hour, and the same 
volume of pure oxygen returned thereto ; and this amounts 
to 7.5 liters per day, and 1125 liters per season for the same 
area. These figures are for plants out of doors in summer; 
for greenhouse plants in winter they approximate to half 
this amount. It will interest the student to convert these 
quantities into the more familiar terms of square yards, 
ounces, and quarts; and it will prove better yet if he see 
them all actually reproduced before him. Further, for the 
sake of those to whom statistics appeal, more figures may 
be added. In a season an average leaf produces enough 
grape sugar to cover itself with a solid crystalline layer a 
milHmeter thick, which is 40 times thicker than the chloren- 
chyma which makes it ; and in the process it absorbs enough 
carbon dioxide and releases enough oxygen to form a column 
of the same area as the leaf 1.125 meters high; and this is 
all of the carbon dioxide in a column of air 3750 meters or 
2.4 miles high. To balance the oxygen absorbed and carbon 
dioxide released in the respiration of an average man for a 
year, there is needed 150 square meters of leaf area working 
through the summer ; or in other words, to balance his 
respiration for a year a man needs all of the oxygen which 
would be released in a summer by the walls of a cubical 
room of leaf surface 5 meters on an edge. 

We have still to explain why both fight and chlorophyll 
are essential to the photosynthetic formation of grape sugar. 
Before the elements contained in the carbon dioxide and 
water can be recombined into sugar, they must first be 



26 A TEXTBOOK OF BOTANY [Ch. Ill, 3 

separated, in part at least, from their existent unions in 
those substances. But both carbon dioxide and water are 
very stable compounds, and therefore their dissociation or 
separation into their constituent atoms requires the applica- 
tion of much power, the basis of which is energy. This 
energy is known to be suppUed by the sunhght, of which 
the role in photosynthesis is thus explained. Now the 
energy in the light cannot of itself effect this dissociation 
(else obviously no carbon dioxide or water vapor could re- 
main in the atmosphere), and accordingly there is also neces- 
sary some agency by which the energy in the light can be 
applied to the actual work of dissociating or splitting the 
molecules of carbon dioxide and water into their constituent 
atoms. That agency appears to be the chlorophyll, though 
it is not yet certain in precisely what way it accomplishes the 
result. Thus the sun suppHes the energy for photosynthesis, 
and the chlorophyll applies it as power to the actual work. 
This is why both are essential. 

The study of chlorophyll by aid of the spectroscope shows 
that practically only certain red and the blue rays are ab- 
sorbed by chlorophyll from the many contained in the 
white sunhght ; but these are known to be the rays effec- 
tive in photosynthesis. Since those rays are absorbed, they 
do not come to our eyes from the leaves ; but the unabsorbed 
rays, those useless in photosynthesis, reach our eyes in a 
mixture which collectively gives the sensation of green. 
Thus the greenness of vegetation is due to the light rejected 
by the chlorophyll after removal of the rays useful in photo- 
synthesis. 

The photosynthetic formation of grape sugar is often 
compared with a process of manufacture carried on by man. 
The leaf is the factory constructed for the work : the epider- 
mis forms the external walls, giving shelter from weather, 
while the chlorenchyma cells are the working rooms, and 
the veins, with stomata and air spaces, the passages 
for access and removal of materials; the sunhght is the 



Ch. Ill, 3] SYNTHESIS OF FOOD 27 

source of power, and the chlorophyll the machinery by 
which it is apphed to the work : carbon dioxide and water 
are the raw materials, sugar the desired manufactured prod- 
uct, and oxygen an incidental by-product. The comparison 
while fanciful in details, is correct in essentials. 

Grape sugar is, however, not the only food material formed 
in the leaves, for they are also the places of construction of 
PEOTEINS. These are substances of the greatest importance 
in plant life, because they constitute the foundational ma- 
terial of the Hving protoplasm. They are composed of the 
elements of the grape sugar, — carbon, hydrogen, and 
oxygen, — together with nitrogen, sulphur, and phosphorus 
derived from mineral compounds absorbed from the soil and 
brought to the leaves with the water. Proteins, though 
many and diverse, are all constructed from grape sugar 
by chemical addition of the other constituents, — nitrogen 
first, and the others later. Unfortunately we know Httle 
as yet, despite many researches, as to their exact place of 
formation in the leaves, whether in the veins or the 
chlorenchyma. They occur abundantly in the veins, along 
which they are conducted into the stem. Nor is it certain 
whether light is essential to their formation, though the 
evidence seems to show not, in which case the energy needed 
in their synthesis must be supplied by chemical action. 
Probably their formation in the leaves is only a functional 
convenience based on the simultaneous presence there of 
the basal grape sugar and the needful mineral matters, 
brought with the water. These proteins, like the grape sugar, 
move continuously along the veins from the leaves to the 
stems. 

The role of the grape sugar thus formed in leaves is very 
fundamental in plant Hfe. First, from it, or from the pro- 
teins built upon it, plants build, by minor chemical trans- 
formations, their entire structure, and form all of the many 
organic materials in their bodies, as will later appear in detail 
in a separate section. Second, the energy of the sunlight, 



28 A TEXTBOOK OF BOTANY [Ch. Ill, 4 

used in forming grape sugar, does not become obliterated in 
the process, but is simply converted into the latent or po- 
tential form. Thus the grape sugar becomes a store of 
potential energy, which is retained through the later trans- 
formations, and which can be released and rendered again 
active by the process of respiration, as we shall later describe 
in full. Grape sugar, accordingly, and its derivatives are the 
source both of the materials and the energy used by plants 
in their growth and work, or, in other words, are their food. 
Furthermore, since all animals are dependent upon plants, 
either directly or indirectly, for their food, the photosynthetic 
grape sugar is the basal food for all animals also. 

This use of the term plant food may seem strange to those 
who know the common appHcation of the word to the min- 
eral salts taken by plants from the soil. The latter usage, 
though well sanctioned by custom, especially 'm connection 
with agriculture, is physiologically erroneous. Food, in the 
physiology of both animals and plants, is that material from 
which the living body is constructed, and energy obtained 
for its work. It is because the mineral salts of the soil supply 
only an insignificant fraction of the substance of plants and 
none at all of their energy that they cannot be considered 
plant food, while the name belongs properly to grape sugar, 
which supplies both. The popular usage arose before these 
matters were understood, but is too firmly fixed to be changed. 
No confusion can arise if one takes note of the connection 
in which the word is employed. 

4. The Cellular Anatomy of Leaves 

The actual process of photosynthetic food-formation is 
performed in the cells of the leaf, to which we now turn at- 
tention. For this study we use the compound microscope, 
which is the indispensable tool of the biologist, and one of 
the most powerful and perfect of all the exact instruments 
which scientific men have invented to extend the range and 
precision of our limited senses. 



I 



Ch. Ill, 4] 



ANATOMY OF LEAVES 



29 



When the microscope is turned directly upon a leaf, it 
shows little, because the tissues as a whole are opaque. But 
if from a typical leaf a very thin slice or section be cut across 
from surface to surface, it will show under the microscope 
the general aspect presented in our picture (Fig. 8). Promi- 




FiG. 8. — A cross section through a typical leaf, that of the European 
Beech ; greatly magnified. The shaded round and oval grains are green in 
the living leaf. (Drawn, with slight changes, from a wall chart by L. Kny.) 

nent in the view are the three tissues of the leaf, — the abun- 
dant chlorenchyma, distinguished by the presence of chlo- 
rophyll (in the shaded discoid grains of our picture) : the 
veins, compact and without color (of which a large one 
shows, on the left) : and the transparent epidermis, which 
covers both surfaces. Also amongst the chlorenchyma 
can be seen the various irregular and interconnecting air- 
passages. The cells composing these tissues are individually 



30 A TEXTBOOK OF BOTANY [Ch. Ill, 4 

visible, — each a compartment inclosed by a wall and con- 
taining various contents. 

The chlorenchyma cells are inclosed by thin walls, and 
contain three kinds of contents. Most prominent of all 
are the chlorophyll grains, or chloroplastids, discoid in 
form, and uniformly dyed by the chlorophyll, which does not 
occur outside them. These chloroplastids have this great 
importance, that they are the actual seats of the photo- 
synthetic process. Within the same cells occurs also an 
inconspicuous, shadowy-grayish, thin-gelatinous material 
(sho"^vn by a sparse dotting in our picture), the protoplasm, 
the living material which builds all the rest. The proto- 
plasm, which contains the chlorophyll grains embedded 
within it, forms in these cells only a lining to the walls, 
against which it is held tightly pressed by the cell sap. 
This sap is water containing sugar and other substances 
in solution ; and not only does it fill the whole cavity of the 
cell, but is ordinarily under tense pressure, sufficient not 
only to hold the fining of protoplasm against the wall, but 
also to keep the elastic wall itself somewhat stretched. 
The chlorenchyma cells are variously shaped, — spheroidal, 
ellipsoidal, ovoid, cyfindrical, — as our picture shows. 
The cyfindrical shape prevails towards the upper surface, 
where the ce Is occur tightly packed together, forming the 
so-called palisade (as distinct from the spongy) tissue ; and 
thus the greater part of the chlorophyll grains are brought 
towards the best-fighted surface. This is the reason for 
the familiar fact that most leaves show a deeper green color 
on their upper than on their lower faces. 

When a vein is cut squarely across, as shown in our picture, 
its cells appear angular, compact, and colorless. Three kinds 
of cells appear in each vein. Firs', is an outer or sheath 
layer forming the bundle-sheath, large and thick-walled 
with thin protoplasmic fining. When seen in lengthwise 
section they are found to be several times longer than wide. 
They are most developed on the largest veins, thinner on 



Ch. Ill, 4] 



ANATOMY OF LEAVES 



31 



the smaller, and very thin on the ultimate veinlets ; and their 
function appears to be mainly that of conducting sugar from 
the leaf into the stem. Second, within this sheath, towards 
the lower side, occur many small, angular, thin-walled cells 
with protoplasmic linings, which, seen lengthwise, are found 
greatly elongated and crossed here and there by distinctive 
perforated plates (Fig. 106), though in the veinlets they are 
much simpler in structure (Fig. 9). These are the sieve- 
tubes and associated cells, and their function is principally 
that of conducting the proteins made in the leaves to the stem. 




Fig. 9. — A leaf veinlet, in longitudinal section, of Fuchsia globosa ; 
greatly magnified. Above are the tracheids, and below are sieve tubes and 
associated cells, but the sheath cells do not show in the drawing. (From 
Haberlandt's Physiological Plant Anatomy.) 



Third, just above the sieve-tubes lie a number of somewhat 
larger, angular, thick-walled cells, lacking a protoplasmic 
lining; they are found, when seen lengthwise, to run to- 
gether into tubes, which are distinguished by characteristic 
spiral and other markings (Fig. 101), though in the veinlets 
they are only spirally marked elongated cells (Fig. 9). 
The function of these tubes and cells, called respectively 
DUCTS and tracheids, is the conduction of water from the 
stem to all parts of the leaf. Ducts and sieve-tubes, the 
former always above and the latter below, in conjunction 
with the sheath cells, make up the veins, which when large 
contain many of all three kinds, but when smaller progres- 
sively fewer, until finally the ultimate veinlets may consist 
of no more than the equivalent of a single duct and a sieve- 
tube. 

Although every chlorenchyma cell performs photosyn- 



32 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 4 




thesis, and therefore must receive water from a duct and 
transmit its sugar and proteins to bundle-sheath and sieve- 
tube, many of them, as imphed in Fig. 8, stand some 
distance removed from the nearest veinlet. It is known, 
however, that chlorenchyma cells can draw water, and like- 
wise pass soluble substances, from one to another, the physical 

methods whereof we shall 
presently consider. Now 
the distances through 
which this method is 
effective must of course 
be limited, and while no 
exact measurements 
appear to have been 
made, it seems highly 
probable that the size of 
the ultimate areas of 
chlorenchyma inclosed by 
the veinlets (as noted on 
page 18) is correlated 
with the number of chlor- 
enchyma cells which can 
thus effectively obtain 
their water, and remove 
their sugar or proteins, through one another. 

The cells of the epidermis are rectangular in section, 
though when viewed from the surface, they are found vari- 
ously shaped, even to lobed and interlocked (Fig. 10). They 
contain protoplasm, but ordinarily no chlorophyll (in the 
higher plants) ; and their walls, as proved by chemical tests, 
are infiltrated with a special substance called cutin, which 
renders them waterproof. Especially characteristic of epi- 
dermis is the fact that its continuity is unbroken except for 
the stomata, of which a single example appears in our picture 
(Fig. 8, also 22). Stomata, however, which provide the 
entrance and exit for carbon dioxide and oxygen, are by no 



Fig. 10. — Typical epidermal cells, with 
guard cells, in outline, seen from the sur- 
face ; magnified to same scale. On the left 
Allium, on the right Sunflower. 



Ch. Ill, 4] ANATOMY OF LEAVES 33 

means mere gaps in the epidermis, for each is flanked by two 
special cells called the guard cells, which close and open 
the stomatal sHt in ways, and under conditions, later to be 
noted. 

The picture of our typical leaf (Fig. 8) shows that the 
stoma opens into a specially large air space. This space 
is continuous with others, and with passages in a con- 
tinuous but irregular system which ramifies everywhere 
through the chlorenchyma, extending even in thin vertical 
passages (not clear in our figure, though shown by suitable 
sections) amongst the densely packed cells of the upper, or 
palisade, chlorenchyma. Thus every cell of the chloren- 
chjrma is reached by the air system, and therefore can re- 
ceive carbon dioxide from the air; and by the same route 
the waste product oxygen is returned to the atmosphere. 
The air system is not constructed of cells, but is inter-cel- 
lular, being formed by a splitting and separation of the cell 
walls in the course of their development. 

The leaf of our picture happens to possess a smooth 
epidermis, but where trichomes are present the epidermal 
cells can be seen to extend into one-celled, several-celled, 
or many-celled hairs, scales, or prickles. Sometimes the 
chlorenchyma also has part, as with many prickles, in which 
case the structures are called emergences. Some of the 
cells inside the leaf, as shown by a single example in our 
picture (Fig. 8), contain crystals, which are excretions, or 
matters useless to the leaf and thus disposed of; and such 
single specialized cells are called idioblasts. 

The mechanism of the leaf as a photosynthetic organ for 
the production of food sugar from carbon dioxide and water 
is sufficiently well known to permit its representation by a 
diagrammatic plan, as given herewith (Fig. 11). The student 
should now understand the process so well that with a good 
section of leaf before him, perhaps aided by our diagram, he 
can see it proceeding as clearly in imagination as he could 
with the physical eye were he sufficiently small to wander 



34 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 4 



at will through the intercellular passages, and view the opera- 
tions ^through the crystalhne walls of the cells. Thus he 
would see the water streaming in continuous current through 
the ducts of the veins to the veinlets, and spreading thence 
from cell to cell through walls and protoplasm until it satu- 
rates every chlorophyll grain. Simultaneously the molecules 
of carbon dioxide are moving in through the stomata and 




Fig. 11. — Plan of the leaf as a photosynthetic mechanism. The chloro- 
phyll grains (darkest shaded) are embedded in protoplasm (lighter shaded) ; 
the water (horizontal lines) is brought by the duct (which lacks proto- 
plasm but has a spirally-thickened wall) , and saturates every part of the leaf, 
sap-cavities, and walls, except the outer walls of the epidermis ; the sugar 
(crosses) and proteins (crossed circles) are removed in the protoplasm-lined 
sheath and sieve cells ; the air-passages ramify to every cell, and open 
through the stomata to the atmosphere. 

along the air passages, then through walls and protoplasm 
to the same chloroplastids. On these green plastids falls a 
flood of white sunhght, from which the chlorophyll stops the 
effective red and blue rays, and turns their vibratory energy 
against the assembled molecules of carbon dioxide and water, 
which are thereby dissociated or shattered into their con- 
stituent atoms, with an immediate recombination thereof 
into grape sugar and free oxygen. The molecules of the 
sugar, dissolved in the omnipresent water, diffuse from cell 
to cell through protoplasm, walls, and sap to the nearest 



Ch. Ill, 5] PROTOPLASM 35 

veinlet, of which it enters the sheath cells and there pas^s 
along the veins to the stem, while the proteins in hke manner 
pass into and along the sieve-tubes. Meantime the mole- 
cules of oxygen are moving out of the chloroplastids through 
protoplasm and wall to the nearest air passages, and along 
them to the stomata and the external air, passing the entering 
carbon dioxide en route. The movement of these materials 
in their paths is of course impelled by definite and adequate 
forces, and the mechanism is capable of continuous action, 
which proceeds without break so long as the conditions remain 
favorable. Meantime something similar, as to the details 
of which we are ignorant, must be happening in the synthesis 
of proteins. That is what every green leaf is doing every 
bright day through the summer. 

5. The Characteristics of Protoplasm 

All study of physiological processes leads directly to pro- 
toplasm, the living part of the organism. It is a perfectly 
definite material, with distinctive appearance and properties, 
and it alone, of all the innumerable materials or substances 
in nature, is alive. In Huxley's famous phrase, protoplasm 
is the physical basis of life. 

Despite its importance, the protoplasm of plant cells has 
an appearance so inconspicuous as to make it most difficult 
either to describe or to represent in pictures. Therefore 
in order to understand it, one must see the material for 
himself in the laboratory. 

In most plant cells, as in those of the leaf lately studied 
(page 29), the living protoplasm is rendered almost in- 
visible by the thicker and denser walls which inclose it. 
However, many epiderriial hairs have walls so transparent 
as to show the protoplasm clearly, in which case the mi- 
croscope reveals an aspect like that of the accompanying 
picture (Fig. 12). The protoplasm here extends not only 
as a fining around the walls of the cyfindrical cell, but also 
in irregular threads across the sap cavity. Protoplasm in 



36 



A TEXTBOOK OF BOTANY [Ch. Ill, 5 



this state has an appearance and texture which most ob- 
servers agree in Hkening to a jelly, a rather thin and clouded 

jelly, which holds various 
small solid bodies, mostly 
food grains, in suspension. 
Scientifically, its constitution' 
is described as colloidal. In 
the oldest cells it often be- 
comes even more thin and 
watery than here, though 
hardly ever a true fluid; 
and the clouded appearance 
often vanishes, leaving the 
protoplasm nearly transpar- 
ent, in which case \i is almost 
completely invisible unless 
killed and dyed by special 
stains. In much younger 
cells, it is more viscous, be- 
coming a gelatinous solid; 
and in resting seeds and 
buds, which have given up 
most of their water, it be- 
comes even as firm in tex- 
ture as dry gelatine or horn. 
Since some of the food parti- 
cles have a yellowish tint, a 
large mass of such proto- 
plasm has a distinctly yellow 
color, as seen in the young 
growing tips of roots, or the 
central parts of young ovules. 
There is usually an obvious 
relation between the condi- 
tion of the protoplasm] in 
these respects and the function of the cell. 




Fig. 12. — The appearance of the 
protoplasm in a typical hair-cell of 
a Gourd, as seen projected against 
a black background ; greatly mag- 
nified. (Reduced from Sachs, 
Lectures on the Physiology of Plants.) 



Ch. Ill, 5] PROTOPLASM 37 

A characteristic feature of the hving protoplasm in plant 
cells is its streaming, manifest by a steady movement of 
the included particles which obviously are carried along 
passively by currents of the protoplasm itself. In some 
cells, especially the very large ones of certain Algae, the 
streaming is so active, even up to 10 millimeters per minute, 
that the protoplasm seems literally to rush across the field 
of a high-power objective, while in others, and especially in 
young cells completely filled by the protoplasm, special 
methods are required for its detection ; and all intermediate 
degrees occur. The streaming is maintained by energy re- 
leased from food by the protoplasm, and apparently it serves 
to promote the commingling and transportation of substances 
throughout the cell. 

Thus ^it is evident that protoplasm possesses no visible 
mechanical constitution such as might be anticipated in 
so remarkable a material. But what is its real ultimate 
constitution or texture, which cannot be as simple as it looks ? 
The exceptional interest of this problem has stimulated the 
most profound researches, supported by the most refined 
methods, but as yet without satisfactory result. It was 
formerly thought, from the appearance of material which 
had been killed, stained, and sectioned, that the working 
protoplasm consists of a tangle of flexible fine fibers holding 
the food granules and various fluids in their meshwork. 
Later researches, however, seem to show that it has rather 
the nature of a foam or emulsion, commonly obscure but 
demonstrable by special methods, in which small globules 
of various dimensions and different materials are suspended 
and held apart by thin films of a certain continuous sub- 
stance ; while variously intermingled are food granules, and 
other small bodies of uncertain significance (Fig. 13). Proba- 
bly the usual ground structure of most protoplasm is thus 
ALVEOLAR, though it develops fibrous elements on occasion. 

Thus the physical structure of protoplasm, in so far as 
known, gives httle clew to the source of its remarkable 



38 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 5 




powers. Its chemical composition, however, is more il- 
luminating, for research has shown that protoplasm is not 
a single substance, but a mixture of many, numbering dozens 
in even the simplest known organisms 
(Fig. 14). These substances are vari- 
ous in complexity, from the simplest 
inorganic salts, through the sugars 
and other carbohydrates, to the dis- 
tinctive proteins, which include the 
most highly elaborate and unstable of 
natural chemical compounds. The 
proteins, indeed, seem to represent the 
essential basis of the protoplasm, the 
other substances being more or less 
secondary or incidental. These many 
substances, some of which would react 
T\ath one another, obviously cannot 
exist heterogeneously intermingled 
within the same solvent, but must 
occur in some definite organization. 
Herein, probably, is to be found the 
significance of the emulsion or alveolar structure of proto- 
plasm, wherein the different substances are kept apart in 
their own separate globular compartments by the neutral 
continuous substance, w^hich permits, however, upon occa- 
sion, those regulated interminglings and reactions upon 
which depend the vital phenomena. At least it seems very 
clear that most of the physiological powers of protoplasm rest 
far more upon a chemical than a physical basis. 

This consideration of the chemical constitution of proto- 
plasm inevitably raises the question, — is there among its 
chemical substances some one which is the distinctive living 
substance and to which all the others are subordinate, or 
do the vital powers inhere in the organization of the mixture, 
no one constituent being itself alive ? We do not yet know. 
Both views have their advocates. The former fits best with 



Fig. 13. — Protoplasm 
from the hair cell of 
a Alalva, showing -n-ith 
unusual clearness the 
alveolar structure ; very 
highly magnified. (Re- 
drawn from Btitsclili, 
Microscopic Foams and 
Protoplasm.) 



Ch. Ill, 5] 



PROTOPLASM 



39 



the vitalistic conception of organic nature held by some 
biologists, and the latter with the mechanistic conception 
held by others. 

Protoplasm is unique in possessing simultaneously two 
sets of properties, phj^sical and physiological. Its physical 
properties, — color, den- 
sity, weight, hardness, etc., 
— are of course simply the 
aggregate of the proper- 
ties of its many con- 
stituent substances. Its 
physiological properties 
are those which are pecul- 
iar to itseK as the living 
material. They are mani- 
fest most clearly in the 
physiological processes of 
plants which they make 
possible; and we need 
here but give, for the 
sake of completeness, and 
rather for future reference 
than present learning, the 
mere roll of their names, 
viz. automatism, regula- 
tion, metabolism, mobility, division, growth, irritability, 
heredity, variability, morphological plasticity. 

All protoplasm originates, and therefore all organisms 
arise, in only one way, so far as known, and that is by 
growth and division (or reproduction) of preexisting proto- 
plasm. Spontaneous generation, or the formation of 
protoplasm anew, out of non-living materials, is not known 
to occur anywhere in nature ; for all supposed cases thereof 
when investigated by scientific methods have been found 
to be only apparent and not real, as Pasteur was the first to 
prove. Thus we can trace back all existent living beings 




Fig. 14. — Portion of the body (Plas- 
modium) of a Slime-mold ; X 225. Such 
organisms, which are naked flat masses 
of protoplasm often several square inches 
in area, provide ample material for chem- 
ical analysis of the substance. (From 
Sachs, Lectures.) 



40 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 5 



in an unbroken protoplasmic succession to the very first 
living organism of the earth. As to the source of the pro- 
toplasm of that first being we know nothing, though we 
have two hypotheses, both of which may be groundless. 
One relies upon an original case of spontaneous genera- 
tion, even though perhaps never repeated. The other makes 

protoplasm itself an evolu- 



Mt* 



til 



--<'.' 






Y--f ; Yh': 






■■-'■-■-ft,., 






^ 



^SFE33l:«t 



tion from earher and simpler 
substances, suited to the dif- 
ferent earlier conditions of 
the earth, and thus carries it 
back to an origin contempo- 
raneous and equi-causal with 
the origin of non-living mat- 
ter. The former is rather the 
mechanistic, and the latter 
the vitalistic view of the 
subject. 

There remains one very 
important characteristic of 
protoplasm, and that is its 
organization within the indi- 
vidual plant or animal. In 
most organisms the proto- 
plasm is subdivided into the 
microscopically small masses 
constituting the cells. This 
subdivision, however, is not complete, for suitable methods 
always show that through the cell walls run protoplasmic 
threads, which, though extremely fine, suffice to keep the 
different cells in physiological continuity (Fig. 15); and such 
threads seem to unite all of the living cells of a plant into 
one protoplasmic system. 

Within each cell the protoplasm shows a definite organi- 
zation, clearly exhibited in typical form in our Figure 12, and 
represented in principle in our generalized picture. Figure 16. 



Fig. 15. — A typical example, in 
Mistletoe, of the continuity of proto- 
plasm by threads through the cell 
walls. The walls have been made to 
swell in order to render the threads 
more clearly visible. (From Stras- 
burger, Jost, Schenck, and Karsten, 
Text-book.) 



Ch. Ill, 5] 



PROTOPLASM 



41 



Most abundant, though often not most prominent, is the 
gelatinous-mobile cytoplasm, which is clearly the working 
part of the cell, — that which transports materials, builds 
the wall, produces chemical reactions, and the like. Next in 
prominence is the nucleus, a rounded body of denser but 
still gelatinous, or colloidal, consistency, lying in the cyto- 
plasm. It seems clearly the control organ of the cell, exert- 
ing upon the work of the cytoplasm an influence which 
guides the building of the organism along the general lines 
of its heredity. Inside the 
nucleus is often a smaller 
NUCLEOLUS, which con- 
sists of a store of nutritive 
matter used by the nu- 
cleus. Third in promi- 
nence in most plant cells 
come the plastids, em- 
bedded in the cytoplasm, 
also of denser gelatinous 

consistency, with rounded Fi^. 16. - A generalized plant cell, show- 
Or discoid forms. They i^S ^^^ constituent parts, in optical sec- 

serve as seats of food for- 
mation, the most prominent kind being the chloroplastids. 
In some cells also, a fourth protoplasmic structure has been 
newly recognized, viz., the very minute elongated bodies 
called CHONDRiosoMES or mitochondria, as to the nature 
of which, however, we as yet know little. 

Such are the protoplasmic parts of the typical plant cell. 
In addition, most cells possess a firm wall, built by the 
cytoplasm, and composed of a firm-elastic water-permeable 
substance called cellulose. The wall has the obvious func- 
tion of a support to the protoplasm, which is far too soft to 
support itself ; and the collective walls of all the cells con- 
stitute a firm skeleton for the plant. In young and small 
cells the protoplasm completely fills the space within the 
wall, but as they grow older and larger, rifts, filled with sap, 




42 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 5 



^ 



n 



u 



Q O 



D O 

o 
O O 



A 



1/ 



appear in the cytoplasm, and these rifts enlarge and run 
together until they form a single great central sap-filled 
cavity ; and thus the cytoplasm is left as a thin lining inside 
the wall, against which it is held tightly pressed by the pres- 
sure of the sap. Obviously the arrangement is one which 
gives a maximal spread of surface with the minimal amount 
of protoplasm; but spread of much surface is an obvious 
functional need of an organism which has a mode of nutrition 

requiring extensive expos- 
ure to light, and a wide 
range in the air and the 
soil. Within the sap cav- 
ity occur also various cell- 
contents, — food grains, 
special secretions, crystals, 
and others, — according to 
the respective functions of 
the cells. 

The details of cell struc- 
ture, especially the shape, 
size, thickness, and compo- 
sition of the wall and the 
character of the contents, 
are most diverse in dif- 
ferent tissues, though ex- 
hibiting usually an obvious 
relation to the particular 
functions of the respective 
parts (Fig. 17). This rela- 
tion between structure and function becomes even clearer 
when the study is extended to animal cells, which also are 
protoplasmic ; for here the cell construction is dominated by 
the very different habits of animals, which are freely and 
actively locomotive instead of sedentary and passive. The 
protoplasm of animals and plants is, however, the same in 
all essentials, and the organisms are so different only because 



TT 



Fig. 17. — Generalized outlines of the 
principal shapes of cell walls of plants. 
They are all derivable, by more rapid 
growth in particular parts of the wall, 
from the small spherical form in the 
center. With these shapes occur all 
degrees of thickening of the walls. (Re- 
duced from Ganong, The Living Plant.) 



Ch. Ill, 61 TRANSPIRATION FROM PLANTS 43 

of their very different habits, centering especially in their 
different ways of acquiring their food. 

6. The Water Loss, or Transpiration, from Plants 

A special feature of the physiology of leaves, and other 
green tissues, is the constant loss of water therefrom to the 
air, — a matter which profoundly influences the forms and 
distribution of plants. It is called scientifically transpira- 
tion, and the student should not permit the resemblance 
between this word and respiration to confuse in his mind the 
two processes, which are wholly unrelated. 

The general fact that much water evaporates from plants is 
well known to all who grow them. The rapid wilting of shoots 
when cut but not placed in water, is visible evidence thereof. 
The water which gathers in drops on the glass covers of ferner- 
ies, or on windows in which house plants are kept, has mainly 
this origin, though of course it comes partly from wet soil. The 
reality of the transpiration from the green parts, as distinct 
from evaporation from the soil, can be shown very perfectly 
by the arrangement pictured herewith (Fig. 18) ; for only the 
leaves and stem are inside the closed chamber, the pot and soil 
being excluded by a special glass plate. Within a few minutes 
some water appears on the glass, at first as a faint vaporous 
cloud, and later in large drops which run down the sides. 
Thus we have a perfect demonstration of transpiration, or 
the removal of water as vapor from leaves and young stems. 

The precise amount of transpiration can be determined 
in several ways, but most accurately by weighing, which 
requires potted plants. To secure transpiration without 
evaporation from soil and pot, we use the arrangement shown 
in our picture (Fig. 19). When a plant thus prepared is 
weighed at intervals on a good balance, the transpiration is 
determined exactly, and since the cover may be raised and 
known quantities of water added at intervals, the experi- 
ment may be continued as long as desired. By this method 
it is found that Hving green parts in the light never wholly 



44 



A TEXTBOOK OF BOTANY [Ch. Ill, 6 



cease transpiration, though its amount may be insignificant, 
while it ranges all the way up to above 250 grams per 
square meter of leaf area per hour. The conventional con- 
stant (page 25) for 
greenhouse plants 
is 50 grams per 
square meter per 
hour by day, and 
10 by night, or 
30 night and day 
together, or 720 
grams per 24 hours. 
This amounts to 
108,000 grams per 
season, which 
equals a layer of 
liquid water all 
over the leaf some- 
what more than a 
decimeter deep; 
and presumably 
this figure will 
prove higher for 
plants out of doors 
in the summer. If 
one can see the 720 
grams transpired 
in 24 hours stand- 
ing in a measuring 
glass in the center 
of a square meter 
of surface, he will 
realize better the most strildng fact about transpiration, — 
its remarkably large amount. All of this water, it must be 
remembered, has to be absorbed by the roots from the soil, 
and hfted through the stem. 




Fig. 18. — A conclusive demonstration of trans- 
piration ; X h The bell jar was dry when placed 
over the plant. Its bottom is a plate split and 
perforated in such a way as to fit closely around 
the stem of the plant. 



Ch. Ill, 6] TRANSPIRATION FROM PLANTS 



45 



Little less surprising than the copiousness of transpiration 
is the variabihty in its amount. Much depends upon the 
character of the plant, for, in general, thick-leaved compact 
kinds transpire less than thin-leaved open sorts, and hairy- 
less than smooth kinds, and slow-growing less than quick- 
growing, though occasional surprising exceptions to these 
rules occur. 
But it also 
varies greatly 
at different 
times in the 
same plant, as 
shows very 
clearly when 
a plant is 
weighed fre- 
quently, or 
still better, is 
made to write 
upon a drum 
of a transpiro- 
graph (Figs. 
20, 21) a con- 
tinuous rec- Fig. 19. — A plant prepared for weight-determinations 
orri nf 'i<s n^uTT, °^ *^® amount of transpiration; X J. 

Ora 01 IIS own ^ ^^^ aluminum shell covers the pot, and the roof is 
transpiration rubber, which may be lifted at will for watering and 

day and night "''"*'"^''^^^"^^- 

for a week or longer, — the proper arrangements of course 
being made to insure that all water loss shall take place from 
the plant alone (as in Fig. 19). If simultaneously, whether 
by personal observation or by use of recording meteorolog- 
ical instruments, records are taken of the conditions of 
weather, — temperature, humidity, light, winds, — the reason 
for the fluctuations in transpiration is found. For thus it 
becomes clear that the rate of transpiration is increased by 
light, heat, dryness (of the air), and winds, and is lessened by 




46 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 6 




darkness, cold, humidity, 
and calm. This is assum- 
ing an ample supply of 
water in the soil, under 
conditions for easy ab- 
sorption, since otherwise, 
of course, transpiration is 
mechanically checked by 
lack of available water. 

Thus it is evident that 
transpiration is affected 
by external influences in 
precisely the same way as 
evaporation, thereby rais- 



FiG. 20. — The Transpiro- 
graph ; X h The plant, pre- 
pared as shown by Fig. 19, is 
adjusted on a balance in such 
a way that when it has tran- 
spired one gram of water, that side 
of the balance rises and closes 
an electric circuit. The current 
acts on the electro-raagnet (visi- 
ble in the picture), which pushes 
a pen against the 
revolving time drum 
(shown by the lines 
and letters) , and 
simultaneously re- 
leases from the ver- 
tical tube a spherical 
gram weight, which 
runs through the 
outlet tube on the right and 
drops into the scale pan. The 
latter is thus depressed, breaking 
the circuit, which remains open 
until another gram of water has 
been lost. Compare the record 
in Fig. 21. 

Such a precise and continu- 
ously self-acting instrument is 
typical of those which it is the 
aim of plant physiologists to pro- 
vide for all of the plant processes. 



Ch. Ill, 6] TRANSPIRATION FROM PLANTS 



47 



ing the question as to the 
relation between the two 
processes. While closely 
related, they are not iden- 
tical, as shown by the 
modern studies on rela- 
tive TRANSPIRATION, that 

is, the ratio between tran- 
spiration and the contem- 
poraneous evaporation, as 
determined by suitable in- 
struments. In brief, tran- 
spiration is evaporation 
affected considerably by 
the structure and physi- 
ology of the leaf. 

The profound effect of 
external conditions upon 
transpiration has many 
important consequences. 
Thus, a conjunction in 
high degree of light, heat, 
dryness, and winds, as 
happens at times in our 
gardens, can cause wilting 
in some plants even when 
they have ample soil 
water, because the roots 
cannot absorb, or the 
stems conduct, water as 
fast as transpiration re- 
moves it. In such cases 
a check in the transpira- 
tion, by the coming of 
night or a sprajdng by the 
gardener, is promptly fol- 



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48 A TEXTBOOK OF BOTANY [Ch. Ill, 6 

lowed by a revival of the leaves. It is apparently a similar 
excess of transpiration over absorption or conduction which, 
no matter how abundant the root water, limits the kinds 
of plants we can grow in the dry air of our houses; for 
house plants, as well known, are not so much those we want 
as those we can make grow. It is clearly the defective 
absorption by roots, which absorb slowly at low tempera- 
tures, in conjunction with excessive transpiration, which, on 
bright, dry, windy days in early spring, causes the drying, 
browning, and death in ornamental evergreens ; and likewise 
a wilting, browning (called wind-burn), and death, in the bud- 
ding foliage of deciduous plants. The winter-killing of 
shrubs, as we shall see later, is also largely identical in nature. 
But the effect of Hght, heat, dryness, and winds upon tran- 
spiration shows most clearly of all in the vegetation of those 
parts of the earth where such conditions prevail in conspicu- 
ous intensity, — the deserts. For there, as well known, and 
represented in pictures in Part II of this book, the thin- 
leaved, open types of plants cannot grow at all, and only 
those sorts can manage to exist which are compact and 
thick of texture, or have other transpiration-hmiting fea- 
tures. The aggregate effect is the peculiar and even some- 
what bizarre appearance characteristic of desert vegetation. 
What now is the physiological meaning of transpiration, 
this water-loss which cannot be wholly stopped even though 
at times it endangers the existence of plants, and greatly 
restricts their distribution? The cellular anatomy and 
physiology of leaves give the answer. All chlorenchyma 
tissues are continually saturated with water, the direct evap- 
oration of which is prevented by the waterproof epidermis. 
This epidermis is practically impermeable to the carbon 
dioxide required by the leaves in their food-forming function, 
and also to the oxygen released in that process ; but the access 
and exit of those gases take place through the stomatal 
openings. When these stomata are open for such gas 
passage, however, there is nothing to prevent the water of 



Ch. Ill, 6] TRANSPIRATION FROM PLANTS 



49 



the chlorenchyma from evaporating through them, and it 
does so. The result is transpiration, which is thus primarily 
not a function in itself, but an incidental accompaniment of 
the food-forming process. The formation of a given amount 
of food requires a definite amount of carbon dioxide, and 
this means so much open stoma, and therefore loss of water, 
in definite mathematical 
proportions. 

The stomata are slit- 
like openings which de- 
velop by separation of 
the walls of the young 
epidermal cells. In so 
far as the passage of 
gases is concerned, they 
might to advantage re- 
main permanently open ; 
but in fact they open and 
close, with a proportion- 
ate effect upon transpir- 
ation. The opening and 
closing m each case is 
produced by action of 
two neighboring epider- 
mal cells, speciaHzed as 

GUARD CELLS (Fig. 22), 

of which the walls are so 
thickened as naturally to spring the cells together, thus clos- 
ing the stoma; but the absorption of more water into the 
sap-cavities rounds out the cells and draws them apart, thus 
opening the stoma to a slit, a spindle form, or even, at an 
extreme, to an almost circular opening. Thus the mechanism 
is such that when the cells of the leaf are collectively losing 
water faster than it is restored from the stem, the guard cells 
tend automatically to close the stoma, checking proportion- 
ally the transpiration, while the access of more water to the 




Fig. 22. — A typical stoma, with guard 
cells, of Thymus, seen from the surface, 
and in cross section. The operation of the 
guard cells is explained in the text. (After 
a wall-chart by L. Kny.) 



50 A TEXTBOOK OF BOTANY [Ch. Ill, 6 

leaf, permitting renewed turgescence of the guard cells, pro- 
duces a reopening of the stoma. One other important con- 
dition, however, influences this result. The guard cells, alone 
of the epidermal cells, contain chlorophyll, and hence make 
grape sugar in light ; and a solution of grape sugar, as will 
later be shown, draws water osmotically from neighboring 
cells, thus increasing the turgescence of the guard cells and 
opening the stoma. Accordingly, while the stomata tend to 
close with dryness, so to speak, they also tend to open in 
light, which is the time when carbon dioxide is needed in the 
work of the leaf. These two conditions, however, often oper- 
ate antagonistically, producing irregularities in the action of 
the guard cells. Thus, while their operation can be viewed 
as adaptive in general, it is not so in detail. In this respect 
the stomatal mechanism resembles most other adaptations, 
which, because so many other factors are simultaneously 
affecting the part concerned, can never be perfect. 

Stomata occur chiefly, and in most plants exclusively, 
on the under sides of leaves, in which position a stoppage 
of their openings, and therefore of gas passage, cannot be 
caused by rain. Against this detriment several adaptations 
have been described, though often misinterpreted as a sup- 
posed need for promoting transpiration. Stomata vary 
much in size, extent of opening, and number, ranging from 
all the way up to near 500 per square milhmeter. Their 
conventional constant (page 25) is 100 per square millimeter 
of surface, and their area when extended the widest possible 
would open y^-^ of the leaf surface (Fig. 23). It is at first 
puzzling to the observer, as it long was to botanists, how, 
through so small a total area of opening, a sufficiency of 
carbon dioxide can enter and so much water vapor escape. 
The explanation has been found in a very curious physical 
fact, viz., that the smaller an opening becomes, the more rapid 
relatively (not absolutely) is the passage of a gas through it 
by diffusion, while such passage is also more rapid through 
slit-shaped than through round openings of the same area. 



Ch. Ill, 6] TRANSPIRATION FROM PLANTS 



51 



Therefore the capacity of the small stomatal openings for 
gas passage is far in excess of that implied by their areas. 
The matter becomes clearer from another point of view when 
we note that an ordinary stoma when open presents to a 
molecule of carbon dioxide or water an entrance or exit as 
great as a passage seven miles wide appears to a man. 

While transpiration is thus primarily an incidental accom- 
paniment of photosynthesis, rather than a physiological pro- 
cess in itself, it does have 
functional value in one 
respect. Plants need in 
their leaves, and else- 
where, certain mineral 
matters which are ab- 
sorbed from the soil; 
and these are lifted with 
the water, and left in the 
tissues by its evapora- 
tion. Indeed, the view 
has been held in the past 
that this is the primary 
functional meaning of 
transpiration, its copi- 
ousness being considered 
necessary because of the 
great dilution of the minerals in the soil water. Later evi- 
dence, however, shows that little relation exists between the 
amount of transpiration and the quantity of mineral matters 
found in the plant. Furthermore, an important role has 
been assigned to transpiration in the dissipation of the exces- 
sive energy poured into leaves at times by the strongest 
summer sun, — an amount sufficiently great to work damage 
in the leaf were it not for the cooHng effect of evaporation ; 
and this advantage must be real, even though incidental 
rather than adaptive. Thus it seems clear that transpira- 
tion is primarily an unavoidable though partially controlled 




Fig. 23. — Diagram to show the num- 
ber, and extreme area of opening, of 
stomata, according to the conventional 
constant ; drawn to scale, 100 times the 
true length and breadth. 



52 A TEXTBOOK OF BOTANY [Ch. Ill, 7 

accompaniment of photosynthesis, while secondarily it per- 
forms the functions of lifting the minerals into the leaves, 
and at times of neutralizing excessive solar action upon 
exposed surfaces. 

Connected indirectly with transpiration is guttation, 
frequent in young herbaceous plants. It occurs at those 
times when roots are supplying water forcibly and abun- 
dantly, but transpiration is checked. The surplus water is 
then exuded through water pores (which are modified 
stomata), at the ends of the veins, where it collects in ghsten- 
ing drops, commonly mistaken for dew. The drops can be 
made to appear by experiment, and are often seen in garden 
plants on cool mornings after hot nights, or even on warm 
humid dull days; while often in cool evenings after hot 
afternoons the water drops run down and wet the fohage, as 
familiar in Cannas. In some measure related to guttation 
is the formation of shell-hke ice on the stems of certain 
herbaceous ^' frost plants" in early winter; for the water 
freezes as it is forced from cracks in the dying stems. 

7. The Adjustments of Green Tissues to Light 

Food formation is the first function of plants, and takes 
place only in chlorophyllous tissues under action of light. 
Accordingly it is natural that plants should exhibit special 
adjustments of their green tissues to the sun. 

• Most prominent of such adjustments is the existence of the 
leaf itself ; for the leaf is simply a thin sheet of chlorenchyma 
provided with accessory veins, air spaces, and epidermis. In 
any typical foliage leaf, as observation indicates and micro- 
scopical measurement confirms, the chlorenchyma is re- 
markably uniform in thickness throughout all parts of the 
blade, in which respect it differs greatly from the veins. 
Furthermore, the chlorenchyma of all foliage leaves, no 
matter whether small, as in Mosses, or great, as in Palms, is 
not far from the same thickness. Exact measurements of the 
cross sections of many common leaves show that in different 



Ch. Ill, 7] 



ADJUSTMENTS TO LIGHT 



53 



kinds the chlorenchyma varies in thickness from .09 to .58 
miUimeter, with a mean at .179, and hence a conventional 
constant at .2 millimeter (Fig. 24). This variation, though 
considerable in itself, is yet wholly insignificant in comparison 
with the variation in the sizes and forms of leaves, with which 
indeed it bears no relation. Leaves of evergreen or leathery 
type which seem specially thick, as in Rubber Plant, have no 

thicker ^chlorenchyma, but only a 

thicker epidermis, while the swollen 

and succulent leaves of Century Plant . 

or Houseleek really combine the func- 
tion of storage with that of food for- . 

mation, and hence fall into another 
category. What then determines this 
singularly uniform thickness (or thin- 
ness) of all foliage leaves? The spec- 
troscope, the instrument by which Hght 
can be analyzed with great precision, 
shows that the red and blue- violet 
rays of the sunhght, effective in photo- 
synthesis, are wholly absorbed by a 
layer of chlorophyll, as dense as that 
in the chloroplastids, a fraction of a 
millimeter thick. Accordingly the ordinary chlorenchyma 
can perform its function only when spread out in layers much 
less than a millimeter thick. If the chlorophyll is less dense, 
i.e. if there are fewer granules in the tissue, the effective Hght 
can go deeper, and the green tissue is thicker though paler, 
as in young stems. Furthermore, a stronger hght can pene- 
trate deeper, and hence effectively illuminate a thicker layer, 
than a weak Hght ; and it is a fact that the thicker foHage 
leaves are those which Hve exposed to the brightest sun, while 
the thinner kinds occur on shaded undergrowth plants. 

Second of the adjustments is the existence of the stem, of 
which the wide-branching structure carries the leaves aloft 
and spaces them out in the Hght ; and this, as will later ap- 



FiG. 24. — The actual 
thickness of the chloren- 
chyma of leaves, as seen 
in cross section. 

The upper, one of the 
thinnest, is Abutilon : 
the lower, one of the 
thickest, is Pelargonium : 
the intermediate is the 
average of many kinds. 

(The lines were drawn 
accurately by measure- 
ment on a very large 
scale, and reduced pho- 
tographically.) 



r; 



54 A TEXTBOOK OF BOTANY [Ch. Ill, 7 

pear, is the primary function of the stem. It is true, not 
all leaves thus attain full individual exposure to Hght, and 
many are shaded more or less by others ; but within certain 
Hmits this does not matter, for the reason, fully proved by 
experiment, that a bright diffused hght is quite as effective in 
photosynthesis as direct sunlight, which contains in summer 
more energy than leaves can utihze. 

Third of the adjustments is the presence of chlorophyll in 
all practicable hghted parts. While leaves are preeminently 
the chlorophyll-exposing organs, this function is by no means 
restricted to them, but is shared in lesser degree by young 
stems, young fruits, and even parts of the flower, though the 
showy corolla and ripe fruits have other colors suited to their 
special functions. It looks as though the plant took ad- 
vantage of all its surfaces not needed in other functions to 
spread to the light such chlorophyll as it can, even though 
that be Httle. 

Fourth of the adjustments is the existence in plants of a 
remarkable property of turning their green parts to the Hght, 
no matter from what direction it comes. The fact is famihar 
in house plants, which turn leaves and stems away from the 
darker room towards the lighter window to a degree pro- 
foundly affecting their forms, while the same power can be 
proved in many striking ways by simple experiments (Fig. 25). 
The younger parts of stems bend over until they point 
towards the light, carrying with them the young leaves, which 
independently set their blades at right angles to the light. 
This bending is effected by growth, which becomes more active 
on the side necessary to swing the stems to the light, and in 
those parts of petioles necessary to swing the blades across 
the light. Obviously the hght does not effect the bending, 
for that is accomplished by the plant through its own dif- 
ferential growth ; but the growth is made in response to 
the greater intensity of the light, which therefore acts 
as the STIMULUS to the bending. This process is called 
PHOTOTROPiSM (formerly hehotropism), and it is typical of 



Ch. Ill, 7] 



ADJUSTMENTS TO LIGHT 



55 



a great many physiologically advantageous adjustments 
which individual leaves, stems, roots, flowers, and other 
organs of plants make not only toward light, but towards 
gravitation, moisture, chemical substances, and other ex- 
ternal influences. This very important property of respond- 
ing thus to external stimuli is called irritability (page 39). 




Fig. 25. — A Fuchsia grown for a week in a box open only on one side ; 
seen in profile and face view ; X j. Traced from photographs. 

Though it often simulates intelligent action, for which it is 
sometimes mistaken by the beginner in these studies, it has 
really no direct relation to the consciousness of animals. 
It does, however, correspond closely with the reflex action 
of animal physiology, each irritable, like each reflex, reaction 
being perfectly specific and invariable in a given part to a 
given stimulus. Being thus, in any given case, automatic, 
these responses are properly describable as self-adjustments. 



56 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 7 



The phototropic response of leaves and stems to light, or 
of any other parts to a stimulus, involves the cooperation of 
four factors. First, there exists in the plant an hereditary 
property by virtue whereof the plant makes the responses, 
which are usually adaptive and evidently acquired in evolu- 
tion in the same way as other plant-features. Second, there 




Fig. 26. 



A leaf-mosaic in English Ivy. (After Kerner, Das Pflanzen- 
leben.) 



is some mode of perception of hght by the plant, the quantity 
of light needed being extremely small, only enough, indeed, 
to make a physical impression upon the sensitive proto- 
plasm. Probably most of the protoplasm of leaf and stem 
is thus sensitive, though special regions are more so than 
others, and various adaptations for concentrating light in- 
side specialized perception cells have been described. Third, 
there is some method of transmission of an influence from 
the perceptive place to a motor mechanism where the actual 
response is produced. This influence apparently travels, as 
a rule, through the protoplasm of the cells and the inter- 
cellular threads (page 40), although special arrangements, 
supposed to facilitate its passage, have also been described. 
Fourth, there is a motor mechanism, resting usually upon 
a differential activity in a growth zone or other growing 
tissue, though in more active responses, as in the Sensitive 
Plant and Venus Fly-trap (page 76), a quick-acting hydraulic 



Ch. hi, 7] 



ADJUSTMENTS TO LIGHT 



57 



mechanism is concerned. It is easy to recognize in the reflex 
actions of our own bodies the corresponding factors and 
mechanisms. 

Since stems and leaves turn usually towards the stronger 
Hght, one may well ask why the vegetation of the northern 
hemisphere does not all bend towards the south. The reason 
seems connected with a fact already mentioned, that leaves 
cannot use all of the energy in full summer sunlight, while 
a strong diffused Hght is enough for their needs. Apparently 
their full power of response is aroused by such diffused light, 
which comes about equally from all parts of the sky. 

Where many leaf blades grow closely together, they tend 
to move out from under one another's shade, their petioles 
bending or elongating in ways which effect this result. Thus 
the blades on a horizontal branch of a tree are commonly 
brought into one flat plane. The effect is particularly strik- 
ing in Ivies, where the leaf blades become often so evenly 
distributed as to suggest the name of leaf-mosaic (Fig. 26). 

A familiar light adjustment is involved in the so-called 
''sleep movements," where the leaflets, of compound leaves, 
as of Clover, Oxalis, Beans, Acacias, Sensitive Plants, droop 
or close together in darkness and spread mdely apart in 
light (Fig. 27). The response to the Hght stimulus is plain, 
but the significance of the 
movement in the plant's 
economy is still uncertain. 
The leaflets of other plants 
exhibit an analogous move- 
ment under very intense 
Hght, in which they close 
together or assume vertical 
positions, returning to the horizontal position when the light 
is less intense ; and this movement has been interpreted as 
protective to the leaf tissues against too intense insolation. 
.A permanent condition of this protective light adjustment, 
which, at its perfection, involves a setting of the leaf edges 





Fig. 27. — Leaf of a Clover, in 
and "asleep" positions. (From Darwin, 
Power of Movement in Plants.) 



58 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 8 



toward the midday sun, produces the '' Compass plants/' 
of which there are several kinds in addition to the more 
famous one of our western prairies. Many other Hght ad- 
justments are also known in nature, not only in leaves and 
stems, but also in roots, flowers, and other parts. They 
include movements towards, from, and variously across the 
Hne of incident light. In many cases, a distinct functional 
advantage to the organism can be clearly perceived, but in 
others this is not evident, though here the limitations of our 
knowledge may be at fault. 



8. The Various Forms of Foliage Leaves 

Foliage leaves are remarkably diverse in their sizes and 
shapes, despite their singularly uniform thickness. They all 

perform the same function, and 
their differences correspond for 
the most part with differences 
in the habits of the plants 
which produce them. 

The sizes of fohage leaves 
range all the way from almost 
microscopic up to that of 
Palms and Bananas, several 
square feet in area (Fig. 28). 
Marshaling sizes against habits 
we find in general that the 
largest leaves occur upon 
plants which have the most 
abundant water and warmth, 
and least exposure to bright 
sun and winds, — in other 
words, upon plants exposed to 
relatively least transpiration. 
These conditions are best realized in the shelter of tropical 
forests, and there we find the largest leaves, as all pictures 
of tropical undergrowth well show (Fig. 29), while the same 




Fig. 28. — The Banana, growing 
12 to 15 feet high, and bearing the 
largest known simple leaves. (From 
Balfour, Class-book of Botany.) 



Ch. Ill, 8] FORMS OF FOLIAGE LEAVES 



59 




Fig. 29. — Primeval tropical forest, in Ceylon. To illustrate the large 
size of leaves in the undergrowth. (Reduced from Kerner.) 

principle holds good in our temperate flora, as the student 
may recall. At the other extreme, very small leaves occur upon 
plants which are exposed to the greatest dryness, brightness, 



60 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 8 



cold, and strong winds, — conditions which make transpira- 
tion excessive. These conditions prevail in highest degree 
in arctic, alpine, and desert regions, and there we find the 
smallest leaves. In our native flora, the same principle is 
exemplified in the plants of bogs, which are open cold places, 
and in the evergreen trees, which have to withstand the rigors 




Fig. 30. — A view in Hawaii, showing the contrast between tall-growing 
compound-leaved and low-growing simple-leaved Palms. (From Bailey, 
Cyclopedia of Horticulture.) 

of winter. Under conditions intermediate between the ex- 
tremes, the leaves are intermediate in size, as our temperate 
vegetation as a whole well illustrates. Correlatively, leaves 
which grow exposed to similar general conditions approxi- 
mate to a similar size, as well shown in our common deciduous 
trees, where the leaves of Maples, Oaks, Chestnuts, Lindens, 
Poplars, and others are not far from one size, or at least be- 
long to the same order of magnitude. 

Leaves whicli are morphologically large sometimes be- 
come physiologically small by compounding of their blades to 
separate leaflets (page 16 ; Figs. 32 and 37). The compound- 



Ch. Ill, 8] FORMS OF FOLIAGE LEAVES 



61 



ing is oftentimes associated with exposure to strong winds, 
as in Palms, where the compound-leaved forms tower high 
over the forests, or grow along wind-beaten strands, while 
the simple-leaved forms are confined perforce to shelter (Fig. 
30) ; and it is probable that the compound leaves of the 
Tree Ferns (Fig. 31) originated in this way. Compounding, 




Fig. 31. 



Alsophila oligocarpa, a tropical Tree Fern, showing the much- 
compounded leaves. (From Bailey.) 



however, has also other associations. Thus, in the Pulse 
Family, it seems clearly connected with the '^ sleep," or 
drooping at night of the leaves. In submersed water plants, 
where it is common, the compounding, by its exposure of 
more surface, facilitates the absorption of the carbon dioxide 
dissolved in the water (Fig. 32). 

While leaf size seems thus largely adaptational, it is 
sometimes as clearly structural or hereditary. Thus the 



62 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 8 



small size of the leaves of Mosses, despite their occurrence in 
protected places, seems structurally determined by the very 
imperfect water-conducting system of those plants. The com- 
pounding, with the consequent small leaflets, of our under- 
growth Ferns seems probably an 
hereditary survival from tree-like 
ancestors. And other minor factors 
enter into these problems. 

In shapes, leaves are equally 
diverse, seeming to defy classifica- 
tion. Yet comparative study re- 
duces them to modifications and 
combinations of three primary forms, 
which are the orbicular, linear, and 
ovate. 

Orbicular leaves are well typified 
b}^ the Garden Nasturtium (Fig. 33), 
with its nearly circular blade and 
central-standing vertical petiole from 
which the veins radiate to the mar- 
gin, giving off a network of veinlets. 
In this leaf the blade is unbroken, 
but in most others a gap or slit runs 
from margin to petiole, as illustrated 
by the Pelargonium (''Geranium"), 
the difference apparently represent- 
ing a different mode of evolution 
from ancestral forms which had mar- 
ginal petioles. Structurally the orbic- 
ular form serves best the leaf function, since it combines 
the most green surface with the least lateral spread, and pro- 
vides the shortest paths of conduction for water and food 
through the blade. Orbicular leaves are found oftenest upon 
low-growing or flat-growing plants, where each blade has 
room for exposure to light unshaded by its neighbors, as in 
''stemless" herbs, in creeping vines Hke Ground Ivy, and in 




Fig. 32. — Bidens Beckii, 
which grows partly im- 
mersed in water and bears 
simple leaves above, and 
compound leaves below the 
surface. (After Goebel, 
Biologische Schilderungen.) 



Ch. Ill, 8] FORMS OF FOLIAGE LEAVES 



63 



leaves which float on the water, as with Water-lilies ; while 
climbing Ivies show the same tendency, usually modified, 
however, by marked angularity of form. The full exposure 
of the round blades to light is aided by adjustments in the 
slender petioles, and it is in such p ants that leaf-mosaics, 
mentioned in the preceding section, become the most perfect. 
Linear leaves are typified by those of the Grasses, with their 




Fig. 33. — Leaves approximating to orbicular shape; X i. Garden Nas- 
turtium, Yellow Water-lily, Pelargonium, English Ivy, Ground Ivy. 

slender elongated blades merging imperceptibly into the pet- 
ioles, and their approximately equal-sized parallel veins 
joined by inconspicuous veinlets (Fig. 34). Such leaves 
occur chiefly in dense growths in the most brightly lighted 
places, either upright and parallel like the Grasses in meadows 
or the Cat-tails along lake sides, in dense radiating heads Hke 
the Bunch-grasses and Spanish Bayonets (Fig. 35), or else in 
mats and tufts, as along the branches of our evergreen trees. 
At first thought it would seem that such leaves, presenting 
their edges rather than their faces to the sun, must be badly 
illuminated. Yet their habitual occurrence in the sunniest 



64 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 8 



places, in conjunction with the daily swing of the sun through 
the sky, must insure among them a sufficiency of that bright 
diffused Hght which, as earher noted (page 54), is fully as 

effective in food 
formation as direct 
sunhght.. Further- 
more, the crowded 
condition of such 
leaves tends greatly 
to restrict tran- 
spiration, without 
equivalent check to 
the access of carbon 
dioxide; and such 
an arrangement has 
obvious advantage 
to plants of limited 
water supply. 

Ovate leaves are 
typified by those 
of Lilac (Fig. 36). 
The petiole, at the 
larger end, merges 
into a strong midrib 
from which spring 
side veins, which in 
turn give rise to a 
network of veinlets. 
This general shape 
is the commonest in nature, and associated with the com- 
monest condition of leaf existence, viz., that in which the 
blades, neither spread out in one plane nor densely crowded 
in full sun, are carried aloft and spaced apart on ascend- 
ing stems and branches, as occurs in our larger herbs, and 
especially in shrubs and trees. This mode of Hfe is essen- 
tially intermediate between that associated with orbicular 




Fig. 34. — Linear and other parallel-veined 
leaves; X |. Hyacinth, Banana (small), Thri- 
nax (a Fan Palm), Eucharis, a Grass. 



Ch. Ill, 8] FORMS OF FOLIAGE LEAVES 



65 



and that with linear leaves, and the ovate shape approxi- 
mates to orbicular at base and hnear at tip. It is therefore 
quite consistent that when the leaves become more crowded 
on the branches, as 
in Chestnut and 
Beech, the ovate 
shape tends towards 
linear, resulting in a 
spindle form ; but 
when on the con- 
trary the leaves are 
more fully spread 
out, the ovate tends 
towards orbicular, 
with the great veins 
coming to radiate 
from an elongated 
petiole, as in Red- 
bud. The tendency 
towards orbicular 
goes farther in heart- 
shaped leaves, Hke 
Linden and Violet, 
and ultimately leads 
back to the true or- 
bicular with central- 
standing petiole. 

Between orbicular, 
linear, and ovate forms, there occur all gradations, giving a 
great diversity of forms. Many of these have been named 
from their resemblance to common objects {e.g. lanceolate, 
spatulate, renif orm, peltate) ; and such designations find con- 
stant use in the descriptions of plants contained in floras and 
manuals. 

Closely connected with the shapes of leaves is their 
VENATION. Orbicular and ovate leaves are typically netted- 




Fig. 35. — Cordyline australis, the "Dra- 
cona Palm," showing radiate heads of linear 
leaves. (From Bailey, Cyclopedia.) 



66 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 8 



veined, that is, have a few prominent veins and many inter- 
secting veinlets (Figs. 2, 33, 36). In the typical ovate forms 
there is commonly one midrib with a few veins running thence 
parallel-diagonal to the margin, and such venation is called 




Fig. 36. 



Leaves approximating to ovate shape ; X h- Lilac, Maple, 
Beech, Redbud, Violet. 



PINNATE, while in orbicular forms several approximately equal 
veins radiate from the petiole, and that is called palmate. 
Linear leaves are typically parallel-veined, that is, have many 
approximately equal veins running parallel, with the cross 



Ch. Ill, 8] FORMS OF FOLIAGE LEAVES 



67 



veinlets almost invisible. In some the veins gradually 
converge towards tip and base, as in Grasses and many 
Lilies ; in others they rmi out strictly parallel from a midrib, 
as in Banana (Fig. 28), while in still others they radiate from 




Fig. 37. — Typical lobed and compound leaves ; X |. Oak, Locust, 
High Bush Cranberry, Virginia Creeper, Orange. The single leaflet of the 
latter is jointed to the petiole, which in related forms bears two additional 
leaflets. 

the base, producing a fan shape, as in the Fan Palms (Fig. 
34). And of course there occur all gradations and com- 
binations. 

There is also close connection between the venation, and 
the lobing and compounding of leaves. Some kinds become 
deeply lobed between their main veins, and therefore pin- 
NATELY LOBED, as in Oak (Fig. 37), or palmately lobed, as 
in Maple. The significance of this lobing is not yet under- 



68 



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[Ch. Ill, 8 



stood, but it seems connected with a tendency of the chloren- 
chyma to collect more closely towards the main veins.. The 
lobing carried farther leads to compounding, which therefore 
is either pinnate, as in Acacia, or palmate, as in Virginia 
Creeper (Fig. 37) ; and often the leaflets are themselves 
compounded, even more than once, as in some Ferns. 
Parallel-veined leaves are rarely lobed or compounded, their 
mode of venation being obviously unfavorable thereto. 
The number of leaflets in a compound leaf can be very great, 
or no more than three, as in Poison Ivy, or even only one, as 
in Orange. 

Leaves differ also in the character of their margins, which 
in some, e.g. Rubber Plant, and most parallel-veined kinds, 

are unbroken or entire, 
but in others are sharp- 
toothed or serrate, e.g. 
Rose, and in others yet 
otherwise formed (Fig. 
38). The differences 
seem to have no func- 
tional significance, but 
represent structural ex- 
pressions of the various 
ways in which the chlo- 
renchyma is arranged with respect to the vein endings. 

Leaves also display some pecuhar forms of tips and bases 
(Fig. 39). The prolonged slender tip found in some leaves 
of tropical plants has been claimed to act as a ''drip point," 
effective in removing water from the leaf after rain, thus pre- 
venting a long closure of the stomata; but the evidence is 
not clear. Some leaves have the base of the blade prolonged 
into ear-shaped (auriculate) or pointed forms, occasionally 
making the leaf arrow-shaped. In some kinds these ex- 
tensions grow together around the stem, which accordingly 
seems to pierce the blade (perfoliate), while in others two 
opposite leaves grow together in similar manner surrounding 




Fig. 38. — Forms of leaf margins. 
(After Gray.) 



Ch. Ill, 8] FORMS OF FOLIAGE LEAVES 



69 



the stem (connate-perfoliate). Such features, for the most 
part, seem to have a structural rather than adaptational 
origin. 

The leaves of plants which grow in places where water 
is scarce or hard to absorb exhibit several features obviously 




Fig. 39. — Special forms of tip and base in leaves ; X ^. Ficus religiosus, 
with "drip" point; perfoliate Uvularia; anriculsbte Magnolia Fraseri; con- 
nate-perfoliate Honeysuckle ; Caladium. 

related to reduction of transpiration. Such are, — reduction 
in size, already mentioned ; compact or rounded forms, often 
storing water, as in Cactus; a very thick epidermis, which 
prevents any loss by direct evaporation ; sunken stomata with 
an air chamber outside, or else inrolled leaves, with the stomata 



70 



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[Ch. Ill, 8 



in the concavity (Fig. 40), or coverings of hairs or scales 

(Fig. 41), all of which arrangements tend to delay the 

escape of water without 
materially affecting the en- 
trance of carbon dioxide : 
and a vertical position of 
the green tissues, which 
lessens the evaporative ef- 
fect of the noonday sun 
without any effect upon 
gas absorption. The collec- 
tive result of these features 
is to give the characteristic 

grayish condensed aspect to the vegetation of dry places. 
The trichomes of plants are indeed remarkable in their 

variety, and often in their beauty when viewed through the 




Fig. 40. — Leaf of Erica, in cross 
section ; X 280. (From Kerner.) 




Fig. 41. — Various forms of epidermal hairs and scales (trichomes) found 
upon leaves ; much magnified. (From Kerner.) 



microscope. Diverse functions have been ascribed to them, 
in addition to their part in restricting transpiration, but 
without convincing evidence. Perhaps they represent a kind 
of play of growth forces rather than any adaptational devel- 
opment. 



Ch. Ill, 8] FORMS OF FOLIAGE LEAV-ES 



71 



A very remarkable form of leaf occurs in the Welwitschia 
mirahilis of Southwest Africa, a plant unique in a great many 




Fig. 42. — Welwitschia (Tumboa) mirahilis, growing in the desert of Kala- 
hari, Africa. The woody trunk, though rnany years old, is but two feet in 
height. (From Kerner.) 

features (Fig. 42). The leaves, only two in number, grow 
at their bases as they die at their tips throughout the long 
life of the plant. 

Leaves are pro- 
duced in buds, but 
produce buds in very 
few cases. The leaves 
of some kinds of Be- 
gonia, however, if cut 
across the veins, de- 
velop buds which 
grow into normal new 
plants ; and gardeners 
are accustomed to 
propagate those Be- 
gonias in that way. In 
the well-known Life 
Plant (Bryophyllum) , 
the rather thick fleshy leaves regularly produce buds at the 
outer ends of the veins (Fig. 43) ; and these buds develop 
freely into young plants when the leaves fall on damp soil, 




Fig. 43. — The Life Plant (Bryophyllum 
calycinum), developing young plants on the 
margin of the leaves ; X |. (From Kerner.) 



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[Ch. Ill, 9 



or even when they are pinned up against a wall in the house, 
as often done for a curiosity. Apparently this leaf is quite 
genuine and not a stem in disguise, as one tends to infer. 

Finally, one often finds fohage leaves which exhibit ab- 
normal features, such as forked, laciniate, crested, or even 
pitcher-form blades, or eccentric coloration, or other unusual 
features. When extreme, such cases are popularly called 
freaks, and in science monstrosities. It happens that mon- 
strosities in leaves are closely connected with those in stems, 
and accordingly we can most conveniently discuss them to- 
gether in a later section. 

9. The Forms and Functions of Leaves other than 

Foliage 

While formation of food is the primary, and usually the 
exclusive, function of leaves some kinds perform addi- 
tional functions, and exhibit corresponding peculiarities of 
aspect and structure. Further, in some leaves the new 

function comes 
to overshadow 
the old, and 
even to replace 
it. In such case 
we have a new 
organ, though 
one which re- 
tains evidence of 
its morphologi- 
cal origin in its 
mode of develop- 
ment, and vari- 
ous peculiarities 
of structure. 

The simplest case of an additional function in leaves con- 
sists in the storage of water or food, the presence of which swells . 
the leaves greatly, as in Century Plant, and Houseleek 




Fig. 44. — M esembryanthemum obconellum, a plant 
which stores water in the pairs of thickened leaves. 
(From Goebel.) 



Ch. Ill, 9] SPECIAL FUNCTIONS OF LEAVES 



73 



(Fig. 44). The chlorophyll, of course, is all near the surface, 
and wanting in the interior cells of the chlorenchyma, which 
increase in number and size, and present a translucent aspect 
if water is stored, but are opaque if much food is present. 
Sometimes the upper parts of the 
leaves become true foliage while the 
bases alone store food, in which case 
these storage parts, after the fohage 
has withered away, form collectively a 
typical BULB, as in Hyacinth (Fig. 45). 
In related plants the specialization 
has gone further, making a division 
'Hjetween fohage and storage leaves, in 
which case the latter become exclu- 
sively food-storing organs, as in the 
bulb scales of Lihes (Fig. 46). 

Another form of food-storing leaves, 
serving also in some cases as fohage 
and in other cases not, are the coty- 
ledons or ''seed leaves" of embryo 
plants, later to be fully described. 

In many kinds of plants, some of 
the leaves deviate in minor features 
from the typical condition, in which 
case they are called collectively 
BRACTS. Commonest of all are the 
little pale scale-like bracts which stand 
under each flower in a cluster, where apparently they have 
no function, but represent foliage leaves in an arrested or 
rudimentary state of development ; for it is a constant struc- 
tural peculiarity of the higher plants that flowers originate 
in the axils of leaves, that is, in the upper angle between 
leaf and stem. Likewise httle scale-like bracts occur just 
below the leaf -like branches of Asparagus and florists' 
Smilax (page 195). In the Linden the bract is much larger 
(Fig. 47), and attached thereto is the flower cluster which 




Fig. 45. — A Hyacinth 
bulb, in section. The 
outer or storage leaves 
are the bases of last 
year's foliage leaves, and 
will be replaced, as they 
wither, by the bases of 
the new leaves surround- 
ing the flower cluster. 
(From Figurier, Vegetable 
World.) 



74 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 9 



grows out of its axil ; while later this bract serves as a ''sail" 
against which the wind acts in transporting the seeds. Very 




Fig. 46. — Various forms of common "bulbs." Nos. 3, Easter Lily, 

4, Jonquil, 6, Lilium pardalinum, and 7, Hyacinth, are true bulbs, i.e. are 
composed mainly of storage leaves. Nos. 2, Colocasia antiquorum, and 

5, Gladiolus, are corms, i.e. storage stems. No. 1, Tuberose, is a tuber, 
and 8, Lily of the Valley, a rootstock, called a "pip." (From Bailey.) 

striking are the cases where the bracts become highly colored, 
thus forming the showy part of a "flower," as in Poinsettia, 

the real flowers of which 
are small and inconspicu- 
ous. The sepals and petals 
of ordinary flowers are also 
morphologically leaves, as, 
in a slightly differeiit way, 
are the stamens and pistils. 
Colored bracts and petals 
retain mostly the structure 
of foliage leaves, excepting 
that the chlorenchyma now 
holds other pigments in 
place of the chlorophyll. 

Another striking case of 
the combination of a new 
function with the old is found in the pitchers and other leaf 
traps in which insects are caught and digested. They all retain 




Fig. 47. —a leaf and 
the specialized bract in American 
Linden. (From Bailey.) 



Ch. Ill, 9] SPECIAL FUNCTIONS OF LEAVES 



75 




Fig. 48. — The Pitcher Plant of Northeastern America, Sarracenia pur- 
purea; X |. 

The frontispiece, reduced, of Barton's 'Elements of Botany (2d ed., 1804), 
the first great American botanical textbook. 



76 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 9 



their chlorenchyma, and the changes are chiefly in form. 
Thus our native Pitcher Plant, or Sarracenia (Fig. 48), seems 
to represent a leaf in which the margin has grown up around 
a central-standing petiole, forming as it were first a saucer, 
then a cup, and finally a pitcher. In the Nepenthes, most 

elaborate of Pitcher Plants (Fig. 49), 
there occurs a partial division of 
labor between the pitcher and foli- 
age functions, for a very perfect 
blade exists in addition to the 
pitcher. Doubt still exists as to 
the precise morphology of the parts 
in this remarkable leaf, though it 
seems most probable that the pitcher 
represents a blade transformed as in 
Sarracenia, with the lid a special 
outgrowth and the seeming blade an 
expansion of the elongated petiole, 
which often serves also as a tendril. 
But we must guard against push- 
ing such homologies too far, be- 
cause leaves and other parts, while 
strongly influenced in development 
by the characteristics of the part 
from which they have evolved, are 
by no means limited to the charac- 
teristics thereof, but often break loose, as it were, and develop 
new features upon their own account. In another well-known 
insect-trapping leaf, that of the Venus. Fly- trap (Fig. 50), the 
morphology is obvious, the petiole becoming expanded much 
like the blade. 

Another function performed by leaves is that of support 
to chmbing plants, in which case they form tendrils, which 
are characteristic organs of most vines. Tendrils are very 
slender almost thread-hke structures, fitted to twine around 
supports, to which they thus attach their plants. In the 




Fig. 49. — Nepenthes, an 
East Indian Pitcher Plant ; 
X §. The slender stalk be- 
tween blade and pitcher 
often serves as a tendril. 

(From Le Maout and 
Decaisne, Traite General de 
Botanique.) 



Ch. Ill, 9] SPECIAL FUNCTIONS OF LEAVES 



77 



simplest case, the petiole acts as the tendril, making a 
turn around the support, as in our common wild Clematis 
(Fig. 51). In other cases, as illustrated by our figures, the 
tendril is a trans- 
formed leaflet or 
leaflets, or else 
stipule-like struc- 
tures, or even the 
entire blade. The 
typical tendril 
moves about 
through the air 
until it touches 
some object; then 
it bends towards 
the touched side, 
and, if the object 
be of suitable form, 
continues the pro- 
cess, and makes '^, 
several turns 
around it (Fig. 
52). Then the in- 
termediate part of 
the tendril be- 
comes twisted to 
a double spiral, 
drawing the plant 
closer to the sup- 
port, after which plant which catches insects by sudden closure of its 
it develops tough '-''* "lades ; X i. (From Figurier.) 

fibrous tissues, thus forming a strong but elastic bond be- 
tween plant and support. In this definite action of tendrils 
we have another instance of those automatic self-adjustments 
made possible by the irritability of protoplasm (pages 39, 
55), this particular form being called thigmotropism. 




Fig. 50. — Venus Fly-trap, Dionoea muscipula, a 



78 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 9 



Another special form and function of leaves is represented 
in the brown bud scales which enwrap the winter buds of 
our trees. They mostly lack chlorophyll, their cell walls 
become thick and well cutinized, and often they develop 



J 




Fig. 51. — Forms of leaf tendrils; X f. Pea, Smilax, Bignonia, Clematis, 
Lathyrus Aphaca. The apparent leaves of the latter are stipules. 

coatings of resin or hairs ; and they fall away as the buds un- 
fold. In some kinds each scale is an entire leaf, in others 
it is a petiole with blade suppressed (Fig. 53), or it may be a 
stipule, as conspicuous in TuHp tree, where together the pair 
forms a close-fitting cap (Fig. 57). 



Ch. Ill, 9] SPECIAL FUNCTIONS OF LEAVES 



79 



Leaves are also often modified to spines, especially in 
plants of dry places. The significance of spines, however, is 
uncertain; for the older 
view that they represent a 
protection against animal 
enemies seems inadequate, 
while the newer idea that 
they result from a struc- 
tural degeneration of leaves 
rendered superfluous by 
changed habit has not won 
acceptance. In the trans- 
formation they lose their 
chlorophyll and flat form, 
and become slender, coni- 
cal, and hard. In some 
cases each spine represents 
a single transformed leaf, 
as is believed true in the 
Cactuses (Fig. 54) ; in 
others they represent the 
midrib and two lateral ribs 
of a leaf, as in Barberry 
(Fig. 55) ; in Euphorbias, 
when paired, they clearly 
represent stipules (Fig. 
57) ; while in some tropical 
climbers the stipular spines 
are very strong downward- 
turned hooks which catch 
firmly upon other vegeta- 
tion. 

While the blade is the 
distinctive chlorenchyma-carrjdng part of the leaf, the 
foliage function is in some cases assumed by petioles or 
stipules, the blade being more or less suppressed. Thus, in 




Fig. 52. — Stages in the twining of a 
tendril, of Bryonia ; X \. This is a 
stem tendril, but the method is the 
same in leaf tendrils. (Drawn, with 
slight alterations, from a wall-chart by 
Errera and Laurent.) 



80 



A TEXTBOOK OF BOTANY 



[Ch. III. 9 




Fig. 53, — Transition from bud scales to leaf, 
showing the former to be petioles, in Box Elder ; 



the Australian Acacias, the chlorenchyma is all in the 
petioles (called phyllodia), which are vertically flattened 
(Fig. 56), while the much compounded blades distinctive of 

Acacias are sup- 
pressed. In other 
cases the stipules 
become enlarged, 
aiding the blade in 
its function as in 
Violets (Fig. 57), 
reaching to a size 
and form identical 
with those of the 
blades as in Gal- 
ium, or replacing 
the foliage altogether as in Lathyrus Aphaca (Fig. 51). The 
causes of these curious substitutions of functions are mostly 
not known, but they are presumably connected with pe- 
culiarities in the past history of the plants. For example, it 
seems likely that the abandon- 
ment of the leaf blade and 
transfer of the foliage func- 
tion to the petioles in Acacias 
represents a mode of adapta- 
tion to a climate increasing 
in dryness. Leaflets, which 
expose much horizontal sur- 
face, are out of place in 
dry climates, while a single 
petiole, flattened vertically, 
is better protected against 
extreme transpiration (page 
70). 

One cannot but notice the diversity of form, and the 
variety of apparent function, in the stipules. In existent 
plants they seem to represent no distinctive organ, but 




Fig. 54. — A cluster of spines from 
an Echinocactus ; X |. (After 
Goebel.) 



Ch. Ill, 9] SPECIAL FUNCTIONS OF LEAVES 



81 



rather a kind of morphological entity easily specialized in 
diverse directions. Recent investigations have shown that 
leaves containing stipules receive from 
the stem three sets of veins, from two of 
which the stipules are supplied, while 
leaves lacking stipules receive but one 
set, or vein. Since the original or primi- 
tive leaf of our modern trees was appar- 
ently three-lobed, the stipules may repre- 
sent the two lateral lobes, which became 
reduced as the middle lobe developed 
into the leaf blade of our existent plants. 
Not all paired structures at the bases 
of leaves are stipules. In Pereskia, a 
climbing Cactus, the 
paired hooks whereby 
the plant clings to a 
support are the first two 
spines of an axillary 
cluster, and in some kinds 
of Aristolochia the leaf- 
like seeming stipules are 
simply the first leaf of an 
axillary branch. In the Telegraph Plant 
(Fig. 58), they are leaflets, much smaller 
than the terminal leaflet ; and in this plant 
they have further the remarkable property, 
that, for reasons uncertain, they are con- 
stantly rising and falling, in short jerky 
motion suggestive of the arms of the old 
semaphore telegraph, — whence of course the 
plant's name. 

Typically, leaves are flat plates of tissue, 
and in heir various transformations this 
plane character is mostly retained. In certain cases, how- 
ever, the face of the leaf develops m_ outgrowth of tissues, 

G 





Fig. 55. — Leaf spines 
of Barberry ; X h 

(After Gray.) 



Fig. 56. — a 
phyllode of an 
Acacia ; X i- 
Often a few leaf- 
lets of the com- 
pound leaves ap- 
pear at the tip. 



82 



A TEXTBOOK OF BOTANY [Ch. Ill, 10 




Fig. 57. — Special forms of stipules 



a kind of branching 
of the face of. the 
leaf. Such seems the 
case in the Hd of the 
Nepenthes pitcher 
earUer mentioned, 
and in the corona, 
or crown, of the 
petals of some 
flowers, notably the 
Daffodil (Fig. 230). 
Thus we see that 
the leaf, though 
having a definite 
and typical primary 
function and struc- 
ture, is yet highly 
plastic in all of its 
features, and can be 
led along many dif- 
ferent Hnes of de- 
velopment. Such 
morphological plas- 



Euphorbia, paired spines : Galium, with two ticitv is character 
opposite leaves simulating a 6-leaved whorl " 



Tulip Tree, bud scales : Polygonum, united in a 
sheath (ochrea) around the stem : Violet, acces- 
sory foliage. 



istic of all parts of 
living beings, and is 
one of their distinc- 
tive properties (page 39). The tracing of such lines of 
development is the distinctive province of morphology. 

10. The Nutrition of Plants Which Lack 
Chlorophyll 

While most plants possess chlorophyll and make their 
own food, there are some which do not. If, now, all plant 
food is based on grape sugar made in green tissues, how 
do these chlorophyll-less kinds secure their supply? The 



Ch. Ill, 10] PLANTS WITHOUT CHIaOROPHYLL 83 

matter is simple; they take it from green plants, or from 
animals which obtain it from green plants. When they take 
it from living plants or animals, they are called parasites, 
the one from which it is taken being known as the host; 
and when they take it from dead plants or animals or decay- 
ing remains thereof, they are called saprophytes. The 
difference between 
parasites and sapro- 
phytes has no par- 
ticular physiological 
significance, but is 
rather a convenience 
in our description of 
those plants. The 
absorbing organs of 
such plants are called 

HAUSTORIA. 

Among the Flower- 
ing Plants, the most 

familiar parasite is Fig. 58. — The Telegraph Plant, Desmodium 
doubtless the Dodder Syrans -, X\. it is native to tropical Asia, but 
■ ""^^ is grown in greenhouses. (From Figurier.) 

(Fig. 59), a relative 

of the Morning Glory. Its slender, orange-colored, smooth 
stem twines around and among various green herbs in the 
fields; and wherever it touches their stems it sends forth 
aerial rootlets which penetrate the tissues until they reach 
the veins (Fig. 59). Here a connection is established with 
both ducts and sieve tubes, from which the parasite can now 
draw both water and food. The most familiar flowering 
saprophyte is doubtless the Indian Pipe or Ghost Plant 
(Fig. 60), the roots of which are beheved to absorb the 
decaying material of green plants, not, however, directly, 
but by aid of a Fungus {Mycorhiza, page 244). Such para- 
sites and saprophytes, having no chlorophyll, need no leaves, 
which accordingly are reduced to mere scales ; and these 
persist only as rehcs of an evolution from chlorophyll- 




84 



A TEXTBOOK OF BOTANY [Ch. Ill, 10 



possessing ancestors. Without leaves, there is small need 
for stems, which accordingly are also much reduced in 
many of the flowering parasites. An extreme in these 
respects is reached in that remarkable flowering parasite, 
the Rafflesia of Java (Fig. 61), where the plant consists 

solely of a single gigantic 
flower (some three feet 
across and the largest 
flower known), which, 
through a very short 
stem and some haus- 
torial roots, is parasitic 
upon overground roots 
of trees. 

The Fungi, including 
the Bacteria, comprise 
many thousands of 
species of parasites and 
saprophytes, which ex- 
hibit structures having 
obvious relation to the 
conditions under which 
those plants live. Para- 
sitic Bacteria mostly 

Fig. 59. -The Dodder, Cz.sc^to^^rop.a; i^^^^i^ ^^^ tisSUes of 

X \. It is here parasitic on Willow, on which living plants Or animals, 

it twines. Note the scale-like minute leaves, r^^^^ i,-„u 4.v.^ „u^^ v^ 

and the flowers in clusters. On the left is a ^ ^^"^ ^^ich they absorb 

section showing the connection of the haus- the nutritive juiceS di- 

torial roots with the veins of the host, ^^^xu^ 4-u„^,,^u +u^ ^.^ii^ 

(From Strasburger.) ^^ctly through the walls 

of their very simple 
bodies. The true Fungi possess no leaves, stems, or roots, 
but consist ordinarily of two parts, — first, a feeding body 
called a mycelium (Fig. 62), composed of numerous fine 
white threads which ramify over and through their hosts, or 
the decaying materials on which they grow; and second, a 
SPOROPHORE which comes out from the surface, and develops 




Ch. Ill, 10] PLANTS WITHOUT CHLOROPHYLL 



85 



the minute reproductive spores in the air where the winds 
can scatter them. Indeed, were it not for the sporophore, 
often the presence of the hidden myceHum would never be 
suspected. The familiar 
mushrooms and molds 
have this structure. 

Parasites, whether flow- 
ering plants or fungi, enter 
and penetrate their hosts 
by use of digestive fer- 
ments, or enzymes, put 
forth by the tips of the 
entering haustoria. En- 
zymes are definite chemi- 
cal substances which have 
power to digest {i.e. con- 
vert into soluble forms) 
the cell walls, starches, 
and proteins ; and these 
digested materials are 
absorbed into the roots 
or mycelium and form 
food for the parasite. It 
is precisely the same with 
saprophytes. The damage 
done by parasites to their 
hosts is of three sorts, — 
first, the removal of food, 
thus tending to starve the 
host plant; second, the 

excretion of injurious or poisonous substances apparently 
by-products of the parasite's own metabolism; and thirdf 
the disturbance of the growth-control mechanism, resulting 
in the production of various monstrosities. 

Parasites and saprophytes are relatively small plants, the 
majority being microscopic; and they constitute an insig- 




FiG. 60. — The Indian Pipe, or Ghost 
Plant ; X y- I* ha'sntro-ehlorophyll, but 
is translucent white in color. (From 
Bailey.) 



86 



A TEXTBOOK OF BOTANY [Ch. Ill, 10 



nificant and inconspicuous part of the earth's vegetation. 
Thus it is clear that their mode of Hfe is far less successful 
than that of green plants. There is, however, another 
group of organisms of similar habit which has been more 
successful in this respect, and that is the animals. They, 
too, are parasitic or saprophytic upon plants, but have 



^^^^^M 


mi^^mmm\>:immmmmmismii 


':^B^:;^mmmmmmi 


^^w^^^^^BSn^K j^^r^^^ssfSS^^K^^?Si^^SStBBtu\ 


HH^^^fl 


^^^Ift^ °'^^^^^HH 


^joB^fl^^^^^^^^^H 


^^^B^t '^^^^^EBlHM^B^PsraHSH 


CT^W 




M 


^^^^^;*rl 




^^^^^^ 


^^^m 



Fig. 61. — Raffiesia Padrna, of Java, parasitic on a root. (From Kerner.) 



this advantage, that possessing the power of free locomo- 
tion, they are not confined for their food to single hosts, but 
can take it from many. 

It might be supposed that in absence of chlorophyll, the 
bright colors displayed by some Fungi, notably the brilHant 
reds and yellows of poisonous toadstools, perhaps have part 
in a food-making process. No evidence for such function 
exists, and the significance of those colors is not known. 

The student may recall that the Mistletoe, a reputed para- 
site, possesses chlorophyll. That plant, however, is only 
a half parasite, for while taking water and minerals from the 
host it makes its own food in its leaves. There are plants 



Ch. Ill, 10] PLANTS WITHOUT CHLOROPHYLL 



87 



which are Hkewise half parasitic upon the roots of other 
plants, as in case of our wild Purple Gerardia. 

Insect-catching plants do not belong among parasites, 
because they all make their own food. The insectivorous 




Fig. 62. — The mycelium (threads ramifying in the ground) and 
sporophores (above the surface) of a small PufT-ball ; X 5. 



habit is connected only with the acquisition of nitrogen 
compounds, as will later appear. 

Finally, there is one other very distinct method of plant 
nutrition. Certain Bacteria which hve in the soil have 
power to make their own food from carbon dioxide and water 
entirely without sunhght, the necessary energy for the pro- 
cess being derived from chemical energy set free by the 
oxidation of substances in the soil. The process is thus 
naturally designated chemosynthesis in distinction from 
photosynthesis. While occurring at present, so far as known, 
in only one group of Bacteria, the method has great interest 
for the reason that it suggests a way in which plants may 
have made their food in the far-distant times before chloro- 



88 A TEXTBOOK OF BOTANY [Ch. Ill, 11 

phyll was developed. The existing chemosynthetic Bac- 
teria, indeed, may represent a survival from that ancient 
epoch, in which case they are doubtless the most ancient type 
of organisms now inhabiting the earth. 

11. The Autumnal and Other Coloration of Leaves 

The distinctive color of leaves is the chlorophyll green, 
which most of them exhibit. Other colors, however, occur, 
especially in ''fohage" and ''variegated" plants, and in the 
autumnal foliage. 

The most prominent of the non-green colors of living leaves 
is red. It is most intense in cultivated plants, such as Japanese 
Maples, Copper Beeches, Coleus, Beets, and Red Cabbages. 
In all cases, however, the color has been greatly intensified 
under cultivation, from a very moderate quantity in the 
ancestors of these plants. Little blotches or streaks of 
red color are indeed very common in wild plants, as in- 
tensive observation, centered on this point, soon reveals. 
The color is due to the presence of a red substance, called 
descriptively erythrophyll but chemically anthocyan or 
ANTHOCYANiN, which is dissolvcd in the sap of the cells. 
Being thus soluble in water, it is easily removed by hot 
water from red leaves, which thereby are left green, showing 
that chlorophyll is present in foliage plants, though masked 
by the more brilliant and abundant erythrophyll. As to 
the reason for its presence, that is greatly in doubt. Prob- 
ably it has no functional utihty in itself, but represents simply 
an incidental product of the complicated metabolism of the 
plant. 

In some cases, however, a functional utility has been 
claimed for erythrophyll. Thus, a great many plants in our 
own flora show in the leaves in early spring a blush of red 
which later disappears. The claim has been made that here 
the red forms a protective screen to the young developing 
parts, by absorbing the blue and ultraviolet rays of the 
sunUght believed to injure unscreened living protoplasm, 



Ch. Ill, 11] COLORATION OF LEAVES 89 

much as the photographer's ruby light cuts off the same 
rays which would spoil his plate in development ; and thus 
is tided over the time prior to the full formation of the 
chlorophyll, which incidentally acts as a sufficient protec- 
tion. It has also been supposed that the absorbed light 
is converted into heat, and used to warm the young parts 
and thus promote their development. The latter explana- 
tion would account for the prevalent red color in the mosses 
of open bogs, which are notoriously cold places. Various 
explanations have also been offered for the deep red of 
the under sides of leaves in some tropical plants, and 
for the brilliant hues of the toadstools. But the evidence 
in these cases does not stand our earher-cited test for sci- 
entific truth (page 13), which shows how much we have 
still to learn about some of the commonest phenomena. 
The case is quite different, however, with the colors in flowers 
and fruits, for here the evidence demonstrates functional use, 
as will later appear. A functional use seems also reasonably 
clear in the beautiful rose-red Algae called ''sea mosses," 
where the red screen (here, however, not erythrophyll, but 
another red pigment) probably aids the underlying chloro- 
phyll in a better utiHzation of the sunlight as altered by its 
passage through the sea water. 

Second in prominence of the non-green colors of living 
leaves is yellow. Indeed, the normal green color of leaves is 
not a perfectly pure green, but tends a trifle towards yellow, 
which, however, is only rarely pronounced in healthy leaves. 
It occurs occasionally in small blotches and stripes in wild 
plants, from which it has been much developed under cul- 
tivation in some variegated leaves, notably in yellow vari- 
eties of Coleus. It is more commonly associated with 
waning vitality of the leaf, whether through old age, or 
insufficient fight, or the action of parasites, or (and above 
all) the fall of the leaves in autumn. It is due to the presence 
along with the chlorophyll, of a mixture of yellow pigments, 
descriptively called xanthophyll, and composed chiefly 



90 A TEXTBOOK OF BOTANY [Ch. Ill, 11 

of two chemical substances, carotin and xanthophyll 
PROPER, though sometimes additional yellow pigments are 
present. Carotin and xanthophyll have the property of 
relatively high stability in hght, on which account they 
show forth in full intensity when the more unstable chloro- 
phyll, which is made only while the leaf is in full health, 
fades away in the light. 

The white colors of leaves represent simply the natural 
color of composition of the leaf structure when all colored 
pigments are absent. The white is translucent in cells which 
contain sap, but is silvery in those which are dead and 
filled with air, as in some variegated Begonias. White 
areas cannot, of course, form food, and are rare in wild 
plants; but they have been greatly intensified in cultiva- 
tion, in the striped and variegated foliage of Begonias, fancy- 
leaved Caladiums, and Ribbon Grasses. Sometimes the 
same leaves contain also areas or stripes of red, thus increas- 
ing the variegation, as occurs very prominently in the re- 
cently-developed Rainbow Corn. 

Various colors appear also in leaves as result of the action 
of parasites, either Fungi or Insects. In some cases the color 
belongs to the parasite itself, as in the Rust of Wheat leaves, 
where it resides in the rusty-red spore masses. More com- 
monly it results from damage done to the complicated metab- 
oHsm of the leaf by the parasite, followed by disappearance 
of chlorophyll, and consequent exposure of the yellow 
xanthophyll; or the tissues may be killed altogether, and 
hence soon display their distinctive decay color, which is 
brown. Colors due to injury by parasites may usually 
be recognized by a certain abnormal or unhealthy aspect 
they give to the leaf, and especially by their wholly irregular 
or asymmetrical distribution in relation to the leaf structure. 

Most striking and interesting, however, of all the non- 
green leaf colors is the autumnal coloration of foHage, which 
constitutes one of the major phenomena of nature. Its 
foundation lies in the fact that with waning vitality, brought 



Ch. Ill, 11] COLORATION OF LEAVES 91 

on by old age or the coming of autumn, a leaf makes no more 
chlorophyll, while that already present fades rapidly away, 
permitting other colors which are present to show, and 
likewise some new ones to form under the altered conditions. 
The rapidity with which chlorophyll can fade in the hght is 
strikingly shown by the simple experiment of exposing a 
fresh alcohoHc solution to strong Hght in contrast with a 
control kept in the dark (page 17). In an hour or two the 
green color is gone, leaving the solution colored yellow by 
the xanthophyll. This experiment shows why leaves turn 
yellow in autumn, for the fading of the chlorophyll exposes 
the xanthophyll, always present with chlorophyll but far 
more resistant to destruction by hght. Thus all autumn 
leaves are yellow, though some acquire additional colors. 
The xanthophyll is easily extracted in a clear solution by 
simply warming yellow leaves in alcohol ; and it is also ob- 
tainable by blanching an alcohohc extract from green leaves, 
as just mentioned. As to the function of this widely present 
xanthophyll (a mixture of carotin and xanthophyll proper), 
that is still unknown, though the constancy of the substances 
indicates some important functional utility. Herein lies 
another of the problems inviting the future investigator. 

Less abundant but more conspicuous than yellow, as an 
autumn color, is red, which is due to the erythrophyll (an- 
thocyanin) already described. Being soluble in the cell 
sap, it is easily removed, in a clear solution, by heating 
the red autumn leaves in water. It is indeed worth one's 
while, for aesthetic as well as educational reasons, to extract 
the green, yellow, and red pigments in their beautiful clear 
solutions, and view them side by side in glass cylinders 
against the light ; for these are the three which give almost 
the entire coloration to all foHage. The erythrophyll origi- 
nates in autumn leaves very differently from xanthophyll, 
for it is not previously present, but is made during the fading 
of the chlorophyll. There is much uncertainty about the 
details, but it seems reasonably certain that it results in- 



92 A TEXTBOOK OF BOTANY [Ch. Ill, 11 

cidentally, as a purely chemical reaction, when certain sub- 
stances, of which sugar is certainly one, and tannin is prob- 
ably another, happen to be present, and, under the conditions 
prevailing in the dying leaf cells, are struck by bright Hght. 
It is the fading away of the chlorophyll which admits 
into the leaf a sufficient intensity of light to produce the 
chemical reaction. That the hght is essential to the process 
is suggested by the extra brilHance of the colors in specially 
bright climates and seasons, and is proven by the fact that 
any leaf which would ordinarily turn red does not do so 
if closely covered by another, as may be tested by experiment. 
Thus red in these leaves does not replace yellow, which is also 
present, but simply outshines it. The reason why some 
kinds of leaves turn red, and others only yellow, appears 
to be simply this, that some kinds contain the necessary 
substances and others do not. It is highly significant in 
this connection that the leaves which turn most brilliantly 
red, e.g. Maples, Oaks, and Sumachs, are noted either for 
their abundance of sugar, or of tannin, or of both. 

Next in importance of autumn colors is brown, which 
has several origins. In some leaves it is apparently an oxi- 
dized product of yellow sap substances called flavone deriva- 
tives ; in others it results from an oxidation of tannins in cell- 
walls when exposed to the light and the air, — precisely the 
same kind of photochemical process which turns wood or bark 
brown with time. In these cases the color has obviously 
no functional utility, but represents a purely incidental 
result of the chemical and physical conditions which pre- 
vail in the dying or dead tissues. When the browning 
takes place not too rapidly, it sometimes combines with 
the yellow of xanthophyll into a beautiful golden bronze, 
as in some Oaks, though it may later become so intense as 
to mask the xanthophyh, which fades slowly, as in Beech. 
With the brown, as with other colors, the exact shade is 
often determined by the simultaneous presence of other 
substances, such as resins, or even by remnants of unfaded 



I 



Ch. Ill, 11] COLORATION OF LEAVES 93 

chlorophyll, or by air-spaces, hairs, or other structural fea- 
tures. In a few cases no brown color appears, and by the 
slow fading of the xanthophyll the tissues are left nearly 
white, as happens to some extent in our Birches. 

All autumnal coloration of foUage rests upon these five 
colors, either singly or in combinations, modified somewhat 
by other substances, or by the leaf structure. The student 
will notice how different they are in their significance to the 
plant, for while chlorophyll has a well-known and vastly 
important function, and xanthophyll an unknown but prob- 
ably important function, erythrophyll and the browns are 
mere chemical resultants of the phj^sical and chemical con- 
ditions prevaihng in dying leaves, and white is the natural 
color of the unaltered leaf structure. In autumn leaves, 
obviously, none of the colors seem to have any functional 
utility to the plants, and autumnal coloration as a whole 
appears to represent simply a gigantic chemical incident, 
comparable with the blue of the sky and the red of a sunset. 
Though thus but an incident, it is a happy one for mankind, 
in whose elevated enjoyment of nature it forms a great 
factor. 

Everybody knows that autumnal coloration is far more 
brilliant in some cHmates and some seasons than others, 
thus showing a marked sensitiveness to external conditions. 
Something depends on the kinds of plants which constitute 
the flora, for plants differ in their susceptibihty to the 
color changes. Again, the coloration is notable only in those 
regions where the transition from summer to autumn is 
rather abrupt, and the vitaHty of the leaves is suddenly 
checked while they are still full of sap ; and it is relatively 
poor in places of gradual transition from summer to autumn 
where the leaves lose their sap before dying. It is through 
the abrupt check to the vitality of the leaves that early 
frosts help the coloring, though they do not cause it, as 
popularly believed. In fact, any cause which hastens the 
waning of leaf vitality brings on the coloration more quickly. 



94 A TEXTBOOK OF BOTANY " [Ch. Ill, 12 

Thus with our Maples, the partial splitting away of a branch, 
an injury to the bark, or infection by disease, will often pro- 
duce the red coloration in the leaves of the injured branch 
while the remainder of the tree is still green. Further, a 
bright cHmate is essential to the best coloration, partly be- 
cause bright light produces a quicker and fuller fading of 
the chlorophyll, and therefore a better exposure of the xan- 
thophyll, and partly because the briUiancy of erythrophyll 
formation is directly proportional to the brightness of the 
light. It is because bright days and frost go together that 
the latter is commonly credited with more than its due 
in the process. The conditions of the preceding summer, 
whether dry or wet, play also some minor part, through 
influence on leaf vitahty. In general, other conditions 
being equal, the brightness of autumn coloration in any given 
region is proportional to the clearness of its autumn climate, 
while its brightness in any given season is proportional to 
the clearness that year. This importance of light explains 
why the color is more vivid in climates hke that of New 
England, where the autumnal skies are prevaihngly bright, 
than it is in old England, where autumn is a season of mois- 
ture and cloud. Finest of all is the coloration in places where 
the summer ends abruptly, the autumn is bright, and the 
frosts come early, as occurs in Eastern Canada, where some 
of us think it is the best in the world. 

12. The Economics, and Treatment in Cultivation, 
OF Leaves 

All cultivation of plants depends for its success upon con- 
formity to their physiological pecuHarities. It is true, 
gardeners and farmers have not had in the past any scien- 
tific knowledge of these matters, but through centuries of 
experience, consisting in observation and trial and the passing 
along of the results, they have reached conclusions nearly 
enough correct for all practical purposes. We consider now 
the practice of plant cultivation with respect to leaves. 



Ch. Ill, 12] ECONOMICS OF LEAVES 95 

Few kinds of plants are cultivated for their leaves alone, 
aside from foliage plants, grown in gardens for ornament. 
Direct utility is confined to a few which happen to store 
food, as in Cabbage, or which contain some palatable relish, 
as in Lettuce, Spinach and other ''greens," or yield some 
special product, hke Tobacco, or serve as fodder for cattle, 
as in Grasses. Such uses, however, are insignificant in com- 
parison with the indirect importance of leaves as the source 
for the food and other useful substances which are formed or 
stored elsewhere in the plant. For this reason leaves, even 
though temporary organs of little direct economic value, 
must all be kept in health and good photosynthetic oper- 
ation; and thereto is much of our gardening and farming 
practice devoted. 

For best health, leaves need ample but not too much sun- 
hght, all the carbon dioxide they can get, plenty of water, 
some mineral salts, and air. 

In winter, greenhouse plants receive little more than a 
fourth of the sunlight of summer, and not enough for their 
needs. Hence house plants must be given the very best light 
available; and good modern greenhouses are studies in 
Hght-efficiency, embodying the best experience and inves- 
tigation in direction of exposure (preferably south or south- 
east), pitch of roof, transparency of glass, and slenderness of 
frame. On the other hand, the full summer sun contains 
not only more energy than plants can make use of, but often 
much more than is good for them, particularly if in green- 
houses, where they lack the free circulation prevailing out- 
doors. On this account it is needful, even in spring, to shade 
such houses by curtains, slats, matting, or paint on the 
glass. Under light thus tempered greenhouse plants grow 
quite as well as in full sunlight, while keeping in better 
general health. Similarly, it has been found that some kinds 
of crops actually thrive better under some shade, though 
this is not wholly a matter of fight, but also in part of 
protection from hail and strong winds. Thus it is found 



96 A TEXTBOOK OF BOTANY " [Ch. Ill, 12 

profitable to grow Pineapples under slat shading in Florida 
and Tobacco under thin cotton tents in Massachusetts; 
while some recent experiments indicate that several common 
crops, including Potatoes, Cotton, Lettuce, and Radish 
likewise do better under some shade. Corn is one plant 
which seems to thrive best without any shade, though it 
is to be noted that this plant exposes not the faces but only 
slanting surfaces of its leaves to the sun. 

The carbon dioxide indispensable to food formation comes 
from the air through the stomata; and therefore the leaf 
must be kept free from dirt which would clog them. Such 
a clogging of the stomata, with consequent starvation of the 
leaves, explains the damage now done to hedges along coun- 
try roads by the dust thrown by automobiles, and likewise 
the death of leaves growing near cement factories, from which 
a very fine dust continually radiates. In minor degree 
dust is a detriment to house plants, explaining the value 
of an occasional spraying or washing by rain, and also the 
following advice contained in a recent almanac, — ''Cover 
your plants kept in the living rooms with a thin cloth when 
you sweep." Not only dust, but the floating spores of plants, 
and also the excretions of some insects, close the stomata in 
greenhouse plants, and necessitate the frequent scrubbings 
which gardeners must give. Fortunately such damage is 
minimized by the fact that most leaves have the great ma- 
jority, or all, of their stomata upon their under surfaces. 

Water is needed by leaves for food-formation, to compensate 
transpiration, to hold the soft tissues tensely spread, and for 
other purposes; and every gardener and keeper of house 
plants knows how essential is an ample supply. In some 
cases, however, no amount of water supplied to the roots will 
compensate the transpiration from the leaves, because of slow 
absorption by roots or transmission by stems. Thus are 
explained several famihar phenomena (page 47), viz. the 
occasional wilting of garden plants when the soil is not dry, 
the limitation in the kinds of plants which can be grown in 



Ch. Ill, 13] USES OF THE PLANT'S FOOD 97 

houses, the disastrous browning, wind-burn, and winter-kill- 
ing of shrubs. One might think it possible to compensate 
these difficulties by supplying water directly to leaves; 
but leaves cannot absorb any appreciable quantity of water, 
and such benefit as seems to follow spraying is due to the 
check in transpiration (page 47). The spraying of plants in 
the sun may even bring damage, because drops of water left 
on the foliage sometimes act as small burning glasses, which 
concentrate the sunhght, kill the protoplasm, and brown the 
foliage in spots. 

Transpiration from leaves has another connection with 
gardening in this way, that seedhngs when transplanted con- 
tinue to lose water; and since the absorbing roots are 
destroyed, the plants always wilt; hence it is best when 
practicable to cover them with boxes, etc., to check tran- 
spiration until new roots are formed. For exactly this reason 
gardeners remove much of the foliage of cuttings before 
placing them in the ground to root. 

Leaves also need certain mineral matters for chemical uses, 
involving the application of fertilizers ; and they must have 
sufficient oxygen, which means fresh air, for their respiration. 
These matters, however, can be considered more conveniently 
in later sections. 

13. The Uses of the Photosynthetic Food 

It has been said more than once in the foregoing pages 
that the photosynthetic grape sugar made in green leaves in 
the light is the basal food of plants and animals alike. Here 
follows the evidence for this statement. 

The photosjrnthetic grape sugar and the associated pro- 
teins move continuously from their places of formation in 
the leaves, and pass along the veins into stems, roots, buds, 
flowers, fruits, and other parts, every cell of which receives 
a share thereof. Within the cells a part of the sugar and 
proteins are chemically transformed into other substances, 
having definite functions in the plant's economy. These 



98 A TEXTBOOK OF BOTANY [Ch. Ill, 13 

chemical transformations are collectively designated as the 
plant's METABOLISM. Functionally, the metabolic changes 
center chiefly in the provision of materials serving five ends, 
— the skeleton, reserve foods, Uving protoplasm, special se- 
cretions, and respiration. 

1. THE PLANT SKELETON. In the great majority 
of plant cells, a part of the food sugar is used in building the 
cell walls (page 41), which collectively constitute the plant 
skeleton. The substance of the walls is primarily cellulose, 
a transparent, elastic, water-absorbing material, of which 
the filter paper of laboratories is a good illustration, though 
cotton and Hnen are nearly as pure. Chemically its formula 
is (CeHioOs)!!, which means that its molecule is composed of 
the combination CeHioOs repeated an unknown number of 
times. The combination CeHioOs (not known to occur by 
itself) differs only sHghtly in proportions from the food sugar 
(C6H12O6 — H2O = C6H10O5), and is clearly transformed 
therefrom. The ease with which cellulose absorbs and trans- 
fers water has high physiological importance in the interior 
of the plant, but would be fatal on the exterior in contact 
with dry air. In these outer walls, however, a part of 
the sugar (or cellulose) is converted into new substances 
called cuTiN and suberin, which are waterproof, and have 
a faintly brownish color ; and the epidermis which enwraps 
the soft parts of plants, and the cork which encloses their 
woody stems, have walls of such cutinized or suberized cellu- 
lose. Furthermore, this cellulose, while ample in strength 
for the construction of small plants, is too yielding for the 
building of large ones, which have to withstand great strains 
from their weight and the winds. Accordingly, in the 
trunks of trees and shrubs some of the sugar (or cellulose) is 
converted into a new substance called lignin, which infil- 
trates and greatly stiffens the walls without loss of their 
power to transmit water ; and such hgnified walls constitute 
WOOD. The shells of nuts, and some coats of seeds, also 
owe their hardness to lignification. And other modifications 



Ch. Ill, 13} USES OF THE PLANT'S FOOD 99 

of the walls occur, including the gelatination familiar in 
the Flax seed, while often the walls are also strongly infil- 
trated with mineral matters. 

The cell walls of a plant collectively form a continuous 
system, somewhat like the cement walls and floors in our 
modern buildings. In the compartments (the cells) lives 
the protoplasm which builds the whole structure. Thus the 
protoplasm, itself too soft and weak to rise from the ground, 
can, like man, construct lofty buildings, in the rooms of which 
it can dwell in the sun. 

It happens that the qualities which fit the cell walls for 
their functions in plants make them also useful to man for 
many of his needs. Hence he appropriates the elastic cel- 
lulose for paper, or, as it occurs in long fibers, for cotton and 
linen to make clothing. The waterproof cork serves to stop- 
per his bottles. The stiff wood provides a rigid but easily- 
worked material which he utilizes, as lumber, for his dwell- 
ings, and as cabinet woods, for his furniture, while it serves 
minor uses innumerable. 

Man makes one other use of cellulose and its derivatives 
not represented by any function in the plant, but dependent 
on an incidental feature of their chemical composition, viz. 
— they will oxidize, or burn, thus providing him with fuel. 
This use goes further than appears at first sight, for coal is 
nothing but the cell walls- of plants which throve in swamps 
of the Carboniferous epoch, and in course of long ages, under 
pressure and warmth, lost the two gaseous constituents, hy- 
drogen and oxygen, retaining only the solid and oxidizable 
carbon, which is the substance of coal. A perfect sequence 
can b6 traced from the photosynthetic sugar made in the 
green leaves of the Carboniferous plants, first to cellulose, 
then in succession, with progressive loss of the gaseous con- 
stituents, to lignin, peat, soft coal, and anthracite. The 
same qualities which make cellulose burn, make it explode, 
in suitable combinations; and hence it is convertible into 
high explosives, useful in peace and deadly in war. 



100 A TEXTBOOK OF BOTANY [Ch. Ill, 13 

2. THE RESERVE FOODS. While much of the photo- 
synthetic sugar is used directly as food by the various living 
cells throughout the plant body, a large quantity is trans- 
formed into reserve materials, which accumulate in special 
parts, to be used later in growth, especially that of the next 
season. The places of such accumulation are buds, bulbs, 
tubers, and seeds; and it is to the presence of these accu- 
mulated foods that the swollen form of those parts is due. 
These reserve foods are of three general classes, — carbohy- 
drates, fatty oils, and proteins. 

The Carbohydrates are minor transformations of grape 
sugar into substances which retain the food value of the 
sugar, though with different physical properties. They in- 
clude the sugars, starches, and hemi-celluloses. 

The Sugars are of several kinds. The photosynthetic 
sugar itself is a mixture of two kinds, grape sugar or glucose 
(also called dextrose) and fruit sugar or fructose, these 
two being the simplest and most stable of the sugars. They 
have an identical formula, C6H12O6, and differ only in the 
arrangement of the atoms within the molecules. Both are 
present, the former more abundantly, dissolved in the sap 
of practically all plants. The glucose, with some fructose, 
accumulates in stems, as in the Sugar Cane, where it con- 
stitutes most of the molasses, and in Corn, whence it is 
taken for use as the clear syrup called '' glucose." Both occur 
also in fruits, where, however, the fruit sugar is usually the 
more abundant ; and they form also the sugar of nectar, which 
is the basis of honey, chief food of many insects. Far better 
known, however, is Cane sugar, or sucrose (saccharose), 
which accumulates in Sugar Cane, Beets, and the Sugar Maple. 
Its formula is C12H22O11, implying a close relation to glucose 
and fructose (2 C6H12O6 -H2O = C12H22O11), to which it is read- 
ily converted back, into a molecule of each, in various ways. 
And several other sugars, differing little from these, occur also 
in plants, though none are especially prominent. Grape and 
fruit sugars can be made artificially in the chemical laboratory. 



Ch. Ill, 131 USES OF THE PLANT'S FOOD 



iQi 



The sugars are very nutritive substances, and thus con- 
stitute reserve food of the highest value to plants. Their 
quahties, however, make them also good food for animals, 
which draw freely upon them. Thus, they form the chief 
food of insects, are an important constituent of the fodder 
of domestic* animals, and give value to the vegetables and 
fruits used by man, who, however, goes much further in his 
utilization of them, since 
he not only systemati- 
cally cultivates and im- 
proves the plants which 
produce them most 
abundantly, but also ex- 
tracts, refines, and stores 
them for his own more 
convenient use. Press- 
ing out the sweet sap, 
he boils away the water, 
obtains the sugar in 
crystals, and refines 
them of impurities, a 
process much easier for 

cane than grape sugar, Fig. 62 a. -starch grains (concentrically 

for which reason the for- striated) in the cells of Potato; highly 

magnified. (From Figurier.) 

mer is common on our 

tables, while the latter is there unknown. Grape sugar, how- 
ever, has another economic importance, in that it is the 
sugar which is fermented to alcohol by the Yeast Plant, 
though that organism has the power first to convert other 
sugars to grape sugar. From this source comes our entire 
store of alcohol, including all of our wines and strong liquors, 
as we shall note more fully in the section on fermentation. 

The Starches, also, originate in transformations of grape 
sugar. Their formula is the same as that for cellulose 
(CeHioOs)!!, with the n signifying a different number. They 
are insoluble in the sap, and exist in the plant as solid grains 




102 



A TEXTBOOK OF BOTANY [Ch. Ill, 13 



(Fig. 62 a), having very characteristic forms and markings, 
differing with the kind of plant (Fig. 63). Starch is 
formed from sugar only in the plastids of the cells, either the 
chloroplastids of the green cells, or the colorless leucoplastids 














Fig. 63. — Typical grains of various starches ; highly magnified. Upper 
row, Potato, Maranta, Pea, Hyacinth ; middle row, Wheat, Oats, Sago, 
Smilax ; lower row, Canna, Corn, Bean, Oxalis. 

The characteristic forms and markings of the grains form invaluable 
identification marks in the recognition of adulterations of foods, etc. (Re- 
drawn from Ganong, The Living Plant.) 

of storage cells; and it cannot as yet be made artificially. 
Starch is particularly abundant in tubers (Potato), tuberous 
roots (Sweet Potato), bulbs (Lilies and Hyacinths), and es- 
pecially in large seeds, to all of which its presence imparts a 
dull, white, firm aspect, in marked contrast to the soft trans- 



Ch. Ill, 13] USES OF THE PLANT'S FOOD 



103 



lucency where sugar is the food, as, for example, in Beets. 
Being insoluble in water and therefore not removable in that 
form from storage cells, starch must be digested before use, 
in w^hich process it is converted by the action of enzymes 
back into grape sugar, the change being marked, as famihar 
in germinating seeds and growing potatoes, by a transition 
from the dull white to a soft translucent appearance. 

Starch, stored by plants for their own uses, forms likewise 
the best of food for animals, which take what they need, and 
like plants digest it by enzymes back 
to grape sugar, in which form it is 
transferred for use to all parts of their 
bodies. It is the principal constituent 
of the ordinary foods of all herbivo- 
rous and graminivorous animals. As 
for man, starch is by far the most im- 
portant of all the food substances 
taken by him from plants. This is 
sufficiently plain when we recall that 
all of the grains, which constitute the 
principal food of the human race, — 
Wheat, Corn, Rice, Barley, Millet, and 
others, — consist chiefly of starch. 

The Hemi-celluloses are much 
less prominent than the sugars and starches. They are 
modified forms of cellulose, having the same chemical for- 
mula, but with the n indicating a different number. They 
occur as extra layers of the cellulose walls (Fig. 64), espe- 
cially in some tropical seeds, which thereby are made heavy 
and hard, as well illustrated in the Date seed, or still better 
the Ivory Nut, — a large seed of a Palm, hard enough to 
serve as imitation of ivory. The hemi-celluloses are easily 
digested by plants but only in part by animals. They merge 
over gradually to the pectins, or fruit jellies (the ordinary 
gelatin being an animal product), which are dissolved out by 
hot water in making preserves, and these again merge over 




Fig. 64. — Thickened 
cell walls (striated) in 
the Ivory Nut. The 
protoplasm (dotted) ex- 
tends into pits persistent 
in the walls. 



104 A TEXTBOOK OF BOTANY [Ch. Ill, 13 

into the gums, like gum arabic, all readily digestible by plants 
and animals. 

The Fatty Oils come ultimately from grape sugar, through 
intermediate stages, including fatty acids. They are really 
mixtures of true fats, which are not volatile, and thus differ 
from the essential oils, to be considered under secretions. 
They are found in a few fruits, such as Olive (yielding olive 
oil), but accumulate in quantity in a good many seeds, from 
which we obtain Castor oil, Cottonseed oil, Linseed oil, and 
some others. They occur usually in small round globules 
among other food substances, giving a characteristic oily 
luster to sections through such tissues, and, while commonly 
liquid, they form sometimes a butter-like solid, as in cocoa- 
butter. They are insoluble in water, and hence not movable 
through the plant until digested back to the soluble fatty 
•acids. Chemically they are rather diverse in composition (a 
typical formula, that of tri-olein, being C57H104O6), but are all 
marked by this peculiarity, — that their proportion of oxygen 
is very small to that of their carbon and hydrogen. 

As with sugars and starches, the fatty oils are also good 
food for animals. They are a valuable constituent of the 
seeds eaten by animals, including man, who also extracts 
and refines them for food and for diverse uses in medicine, 
arts, and manufactures. Like the animal fats to which they 
are so closely related, their paucity of oxygen makes neces- 
sary a large supply of fresh air for their assimilation ; but 
they yield a great deal of heat, which explains why fats are 
so craved in cold climates. 

The Proteins are much more complicated substances, form- 
ing the most important, even if not the most abundant, 
of the reserve foods. While scattered throughout all living 
cells, they accumulate chiefly in seeds, where they occur 
mostly as solid grains, either scattered throughout the cells, 
as in Peas and Beans, or in a special layer just underneath 
the husk, as in Wheat and other grains (Fig. 65). There 
are hundreds of kinds of named proteins, grouped under 



Ch. Ill, 13] USES OF THE PLANT'S FOOD 



105 




Fig. 65. — Section across a 
grain of wheat, showing the layer 
of protein-holding cells under the 
husk and outside of the starch- 
holding cells ; X 180. (From 
Strasburger.) 



certain chemical classes, the chief of which are the albumins, 
material like white of egg, glutelins, in semi-crystaUine 
grains (Fig. 66), globulins, fa- 
miliar in the gluten of flour 
which gives tenacity to dough, 
NUCLEO-PROTEiNS, the chcmical 
basis of the chromosomes (the 
most important part of the pro- 
toplasm), and a great many 
others. While ordinarily in 
soHd grains, they are all digest- 
ible by enzymes into soluble 
and diffusible forms called pep- 
tones and PROTEOSES, and thus 
can be moved through the plant. 
Chemically they are all very 
complex, for to the elements of 
grape sugar there are added small amounts of nitrogen, sul- 
phur, and phosphorus, taken with water through the roots ; 
and it is for this reason that nitrates and phosphates in par- 
ticular are so essential to fertility in a soil. The stages in 
their formation are complicated, and 
only partially known, but it seems clear 
that first the nitrogen is added chemi- 
cally to the elements of the sugar, 
forming amino-compounds or amides 
(containing C, H, 0, N), with which 
later the other elements are combined. 
These amides are inconspicuous sub- 
stances though widely distributed in 
plants, the most common being Aspar- 
agin, C4H8O3N2. There is good reason 
to beheve that many of the proteins are 
built up from a simple combination in 
much the same way that we found the starches and cellu- 
lose are based on a CeHioOs foundation (page 98). These 




Fig. 66. — A cell 
from Castor Bean, 
showing the protein 
grains, of which the 
structure is rendered 
visible by treatment 
with reagents. 



106 A TEXTBOOK OF BOTANY [Ch. Ill, 13 

proteins, composed of the elements C, H, O, N, S, [P], in 
diverse, but always complicated, proportions, form the basis 
of flesh in animals ; and it is because the seeds of the Pulse 
Family (Peas and Beans) contain so much protein that they 
approach near to meat in their food value. 

Like the carbohydrates and fatty oils, but perhaps even 
more than they, the proteins are good food for animals, which 
take them in fodder, vegetables, fruits, and grains. To ani- 
mals, however, they have this special importance, that while 
muscles, nerve substance, and other essential tissues are 
composed chiefly of proteins, the higher animals at least 
have no power to construct them from simpler substances, 
but must take them ready-made from plants, or from ani- 
mals which have taken them from plants. It is for its con- 
densed supply of such proteins that meat has such food value, 
and it is, of course, for their value as protein-accumulators 
from plants on his behalf that man keeps cattle and other 
domestic animals which he eats. Unlike the case of the sug- 
ars, starches, and fatty oils, however, man does not, because 
of practical difficulties, extract the plant proteins and re- 
fine them for use, though he can do so when he wishes ; but 
he usually takes them with the other food materials which 
they happen to accompany. 

3. THE LIVING PROTOPLASM. The living material, 
the most important in all organic nature, has already been 
described (page 35). It is chemically a mixture of a great 
many substances, but its greater and most essential part is 
composed of proteins. The proteins, indeed, have their 
great importance as reserve food because they are a step in 
the formation of living protoplasm. Some of these proteins 
are very complex (one, for example, has the formula 
C720H1134N218O248S5, and much more compHcated kinds are 
known) ; and they are consequently unstable and labile, 
changing into other forms with absorptions or releases of 
energy which are the foundation of various phenomena of 
fife. But our knowledge of the chemistry of the Hving 



Ch. hi, 13] USES OF THE PLANT'S FOOD 



107 



protoplasm is wholly insignificant in comparison with the 
magnitude and importance of the phenomena it displays. 

4. THE SECRETIONS. These are numerous and di- 
verse substances having each a special meaning in the plant's 
economy. Chemically they are as different as well can be. 
Some are carbohydrates ; others are hydrocarbons (con- 
taining carbon and hydrogen only) ; some contain nitrogen 
like the amides ; while still others are obvious transfor- 
mations from proteins. Some secretions have a perfectly 
obvious function ; others clearly have some function though 
it is not known ; but in many cases the substances seem 
to represent simply by-products of functional changes, or, 
like autumn colors, the incidental result of conditions which 
happen to occur in certain parts. Some of them serve well 
certain needs of man, who takes them for his purposes, often 
extracting and refining them to this end. The principal 
classes of secretions are the following. 

The Essential Oils, or aromatic oils, best known in 
Clove oil. Cedar oil, oil of Lavender, and of ^^ Lemon Ge- 
ranium," and the oil of Orange rind, differ greatly from the 
fatty oils in being volatile, 
and hence giving odors. They 
occur in plants in special cells, 
or in special collections of cells 
called glands (Fig. 67). They 
are the basis of practically all 
the odors of plants, including 
the fragrance of flowers, to 
which they serve to guide 
insects in connection with 
cross-pollination, later to be 
more fully considered. In 
leaves they have been sup- 
posed to give protection, by their acrid taste, against insect 
enemies, or to have other uses, for all of which the evidence 
is still insufficient. Chemically they are in part hydrocar- 




FiG 67. — A gland, in section, 
containing ethereal oil, in Dic- 
tamnus Fraxinella; much magni- 
fied. (From Sachs.) 



108 A TEXTBOOK OF BOTANY [Ch. Ill, 13 

bons, or else contain also some oxygen, being formed without 
doubt from carbohydrates. Their pleasant odors and tastes 
are utilized by man in perfumes and essences, though in 
recent times he has been able to dispense with the plants, 
and manufacture a great many in his own chemical labora- 
tory. But they will always continue to add charm to our 
gardens. 

Related to the essential oils are some other substances of 
considerable importance, of which the most important are 
resins, camphor, and caoutchouc. Resins, known to us in 
balsam, rosin, pitch, and spruce gum, are formed mostly in 
special passages, and are particularly abundant in the 
Coniferse or Pine Family; but we know little as to their 
significance, whether functional or incidental. Man utihzes 
their imperviousness to water in various ways. A fossil 
resin is amber. Camphor is a gum of a special tree, again 
of unknown significance, and having well-known uses by 
man. Caoutchouc, the basis of rubber, is formed by many 
plants, usually in their ''milk" (or latex), though its meaning 
to the plant is uncertain ; but the uses that man makes of 
its wonderful tenacity and elasticity need no description. 

The Pigments are the substances which give the bright 
colors to the various parts of plants. They are very diverse 
in chemical composition (often including elements addi- 
tional to those of carbohydrates and proteins), and in 
significance to the plant. Thus chlorophyll (composition 
C54H7206N4Mg) has a function already familiar to the stu- 
dent, while the ever-associated xanthophyll (composition 
C40H56O2) and carotin (C40H56) have, no doubt, a function, 
though it is unknown. Anthocyanin, called descriptively 
erythrophyll (composition, in a typical case, the Cranberry, 
C21H23O12CI) is the basis of the reds, purples, and blues in 
plants, yielding red with acid cell sap, and blue with alka- 
line. In flowers these and other pigments help to guide in- 
sects, and in fruits other animals, for functional reasons 
later to be noted ; but in other cases they seem to represent 



Ch. Ill, 13] USES OF THE PLANT'S FOOD 109 

simply incidental by-products of other processes, as in 
foliage plants (page 88), in autumn leaves, in the heart 
wood of trees, in the colored saps of roots and stems, and 
in the highly colored Fungi, though in some of these cases 
investigators have found suppositional explanations of their 
presence. These pigments are mostly too unstable in Hght 
to serve any useful purpose to man, unless we consider pleas- 
ure a utihty, for he takes great delight in assembHng them in 
gardens. Some pigments, however, are stable, including 
a few which lack color in the plant but acquire it on ex- 
posure to air {e.g. indigo and madder), making them useful 
dyes. But chemists can now make such dyes artificially, 
and more cheaply than we can obtain them from plants. 

The Alkaloids are best known to us in Morphine (from 
the Poppy), Nicotine (from Tobacco), Quinine (from a tree 
bark, Cinchona), Strychnine (from seeds of Nux vomica), 
Cocaine (from the leaves of a shrub, Erythroxylon Coca) ; 
while Caffein or Thein (from Coffee and Tea), and Theo- 
bromine (from the Cacao tree) are related, if not actually 
in the same class. They occur mostly in special cells or tubes 
(often in the ''milk" system, or latex), but their signifi- 
cance to the plant is very uncertain. Some investigators 
hold that they are semi-poisonous waste products which the 
plant thus isolates, while others have thought that their 
powerful bitter tastes form a protection to the plants against 
animal foes. Chemically they are composed of C, H, 0, N, 
thus suggesting a derivation through the amides. They are 
all endowed with active properties, which are the source of 
their value to man, for, as the Hst above given will show, 
they include some of the most efficacious stimulants and 
powerful poisons which are contained in our materia medica. 
In fact, the principal plant poisons and our most important 
drugs belong in this class. The ptomaines, those well-known 
poisons resulting from the action of Bacteria in animal 
tissues, are also alkaloids. 

Related to the alkaloids in their active properties are some 



110 A TEXTBOOK OF BOTANY ' [Ch. Ill, 13 

of the substances called glucosides, a very large and het- 
erogeneous group, probably of diverse significance to the 
plant, characterized chiefly by the chemical fact that they 
consist of glucose (grape sugar) in union with another sub- 
stance. Certain ones give the bitter taste to nut kernels, 
and to the bark of many trees, and the peppery taste to 
Nasturtium, Water Cress, and some other plants. 

The Enzymes are the most important of the plant secre- 
tions. They are formed in small quantities but large numbers 
of kinds in diverse parts of plants, where they are apparently 
dissolved in the protoplasm. Chemically they are supposed 
to be proteins, but this is not certain, for, while we know their 
effects, we hardly yet know the enzymes themselves. This 
is because of the great difficulty of extracting them in a pure 
state from the comphcated protoplasm. Their importance 
depends upon the fact that, Hke the catalyzers of the chemist, 
they cause chemical changes in various substances (each en- 
zyme but one change in one substance, as a rule), without 
themselves entering into the reaction ; and on this account 
very small quantities of enzymes can change great quantities 
of substance. It is apparently by the action of enzymes that 
the majority of chemical changes in plants are brought 
about. Thus an enzyme called diastase is active in diges- 
tion, changing the insoluble starch into soluble sugar both in 
germinating seeds and animal saKva; another, called zy- 
mase, secreted by the Yeast Plant, changes sugar into al- 
cohol and carbon dioxide, as will be described under fermen- 
tation ; lipase converts fats to soluble fatty acids ; pepsin 
changes insoluble proteins into soluble peptones both in seeds 
and the animal stomach; and so with many others. No 
phase of plant chemistry is now of such acute interest and 
active investigation as that concerned with the enzymes. 

Other secretions are the following. The fruit acids, malic 
and citric and others, give the tart taste to fruits, of functional 
utility in connection with dissemination by animals, and pleas- 
ing to man. The tannins occur chiefly in the bark of plants, 



Ch. Ill, 13] USES OF THE PLANT'S FOOD 



111 



where their bitter, astringent taste has been supposed to 
protect the trees against rodents and insects, while a certain 
antiseptic quahty prevents development of parasitic Fungi 
and hence decay of the bark. It is the oxidation changes 
in these tannins under weathering which give the dark brown 
color to old bark. Having incidentally the remarkable prop- 
erty of hardening the gelatine in skins, they are utihzed 
by man for tanning leather, though here again the chemist is 
providing artificial substitutes. The plant waxes occur as 
the ''bloom" upon some fruits and leaves, and at times, as 
in the Bayberry of the coast, such a wax is abundant 
enough to be collected and used for candles, as our forefathers 
found; but the meaning of the wax to the plants is not 
certain. And other secretions occur, of more special kind 
and mostly uncertain significance. 

Rather common in plants are crystals, frequently, though 
not always, in cells differing from their neighbors ; and 
they often exhibit marked 
beauty of form (Fig. 68). 
They are composed chiefly 
of oxalate or carbonate of 
lime, and represent not 
secretions but excretions; 
for they seem to be either 
useless by-products of func- 
tional chemical reactions, 
or else substances brought 
into the plant from the soil 
with the water, and not 
needed in growth. The 
plant has no continuously-acting excretion system such as 
the higher animals possess, but instead accumulates waste 
matters in out-of-the-way cells, often in leaves and bark, the 
fall of which does incidentally provide an excreting system. 

5. RESPIRATION. The photosynthetic sugar has one 
other use, not at all inferior in importance to any yet 




Fig. G8. - - Crystals of calcic oxalate, 
in a cell of Begonia ; much magnified. 

(After Kny.) 



112 A TEXTBOOK OF BOTANY [Ch. Ill, 13 

mentioned, namely, a considerable quantity is consumed 
in RESPIRATION, whereby energy is set free for the work of 
the plant. This important subject will find treatment in 
the next chapter, along with plant growth where its mani- 
festations are plainest. There, also, will be traced the final 
fate of all the plant substances after they have served their 
functions, or played their other respective parts, in the Hfe of 
the plant. 

Thus all of the substances constituting the plant body, — 
the skeleton, foods, living protoplasm, and secretions, and 
also the materials from which is derived the energy by which 
plants do their work, — are built up from the photosynthetic 
sugar, either by direct transformations thereof, or with cer- 
tain small additions from a few mineral substances taken by 
the roots from the soil. Upon these materials made by 
plants all animals are dependent for their food, both that 
from which they construct their bodies, and that which 
yields the energy for their work. Thus the importance of 
the photosynthetic sugar, of the green leaves, and of the 
photosynthetic process becomes abundantly clear. 



CHAPTER IV 
THE MORPHOLOGY AND PHYSIOLOGY OF STEMS 

1. The Distinctive Characteristics of Stems 

Stems are second only to leaves in prominence and im- 
portance as a constituent of vegetation. They are dis- 
tinguished by their tapering-cylindrical, continuous-branch- 
ing, radiate-ascendant forms, so constructed as to support 
and spread the leaves in the hght. This is their primary 
function, although, as with other plant parts, some kinds per- 
form additional and even substitute functions. 

Foliage-supporting stems, even when performing the same 
function, differ greatly in their external features. In shape, 
their differences center in diverse degrees and methods of 
branching, as will later be noted. In size, they range from 
minute in small herbs, all the way up to the gigantic stature 
of the famous California Redwoods (Sequoia gigantea), over 
320 feet tall and nearly 30 feet through, or the Gum trees of 
Australia {Eucalyptus amygdalina) , even taller though not 
so stout. In mere length, however, these stems are much 
surpassed by*the Rattan Palm, which clambers as a vine for 
more than a thousand feet through the tropical woods. In 
texture, all herbaceous stems, including the new growth on 
trees, are soft-cellular like the leaves, being softest in water 
plants, which are supported by their buoyancy in the water. 
In trees, however, the stems become firm in various degrees 
through softwood and hardwood, even to '^ironwood," as 
familiar in lignum vitse. In color herbaceous stems are green, 
from presence of chlorenchyma, which aids the leaves in food 
formation ; but older stems, which develop a thick protective 
I 113 



114 A TEXTBOOK OF BOTANY [Ch. IV, 1 

bark, are brown or gra}^, as the incidental result of the 
weathering-decay of the tissues. 

Stems differ much in duration, according to the habits of 
the plant. Some are annuals, that is, they start from seed, 
develop an herbaceous shoot, use their food to make new seeds, 
and die, all in the same summer. They abound in our flower 
gardens and include most weeds. Others are biennials, 
that is, they start from seed, develop an herbaceous shoot, 
•store food in some underground part, and die to the ground 
in one summer ; then they use this food to form a new shoot 
which develops seeds and dies completely the second season. 
They are familiar in our vegetable gardens, in Beets and 
Carrots. Some are herbaceous perennials, that is, they 
act like biennials except that they continue to form a food 
supply and develop new shoots and new seeds year after 
year. They include most of the favorites of our flower 
gardens. Others again are woody perennials, that is, 
they do not die back to the ground at all, unless accidentally, 
but persist and become woody, so that each season's new 
growth is added upon that of the preceding year, thus de- 
veloping shrubs and trees. Then there are some which, 
like the annuals, flower and form seed only once in their 
lives (monocarpic plants), but take many years in prepara- 
tion. This is the case with the Century plant, which accumu- 
lates food for thirty years or more, then blossoms, forms seed 
profusely, and dies ; but the same habit is found in other 
groups, including even some Palms. • 

The mode of growth of the woody perennials, whereby each 
season's growth is added upon the preceding, involves none 
of the internal limitations of size or age to which animals are 
subject. Hence trees continue to grow until stopped by 
causes incident to their very size, such as the difficulty of 
transferring a sufficient water supply to great heights, and 
the leverage they come to present to the action of storms, 
whereby branches are broken, rot Fungi admitted, and decay 
begun. Trees fortunately constructed in relation to these 



Ch. IV, 2] 



STRUCTURE OF STEMS 



115 



conditions can attain to a great size and age. Thus the giant 
Redwood is known to exceed two thousand years in age, some 
trees now standing being probably older than the Christian 
era, while the Dragon Tree of the Canary Islands has been 
claimed to live even longer. If, however, mere age is in 
question, there are probably much older plants, for the Sphag- 
num mosses of peat bogs appear to have had a continuous 
growth from the inception of the bogs at the close of the 
glacial period, many thousands of years ago. 

Stems, like leaves, perform also special functions, when 
suitably modified in structure, — forming tendrils, storage 
organs, and even fohage, as will later appear. It is easily 
possible, for the most part, to distinguish such stems from 
leaves, — for stems usually grow from buds in the axils of 
leaves, while leaves have buds in their axils. ^ 



2. The Structure of Stems and Support of the Foliage 

The primary function of stems, and their distinctive con- 
tribution to the plant's mode of Hfe, is the support and 
spread of the foliage. Therewith, however, are involved 
minor functions, notably 
conduction of water and 
food, with growth, respira- 
tion, and self -adjustment to 
prevailing conditions. 

Typical foliage-support- 
ing stems are herbaceous 
when young, but commonly 
become woody with age. 

Herbaceous stems, whether 
true herbs or the herbaceous 
tips of woody branches, are 
typically cyhndrical and 
upright, and produce the 
leaves horizontally all 
around. At the tip 




Fig. 69. — A typical leaf^bearing 
stem, of Norway Maple ; X i- (From 
is a Kerner.) 



116 



A TEXTBOOK OF BOTANY 



[Ch. IV, 2 



bud developing the leaves, which are there small and close, 
but which downward are progressively larger and more 
widely spaced apart (Fig. 69). The leaves stand usually 
upon shght annular swellings of the stem, sometimes ob- 
scure and sometimes well marked, called nodes, which are 
separated by smooth cylindrical leafless internodes. In 
the axil of each leaf occurs a small bud, the foundation of a 

branch, which later de- 
velops and bears leaves 
precisely in the manner of 
the main stem. 

In their tissues, herba- 
ceous stems are much like 
the leaves, as to chloren- 
chyma, epidermis, stomata, 
trichomes, and peculiari- 
ties of color. The veins, 
however, do not show to 
the eye, being buried within 
the cyhndrical stem. In 
cross sections cut close to 
the bud one sees little more 
than the general growth 
tissue, but farther back ap- 
pears some such aspect as 
that of our picture (Fig. 
70). Beneath the thin 
epidermis Hes the chlorenchyma, pale green but rather thick, 
obviously aiding the leaves in food formation. Centerward 
can be seen the cut ends of the veins, called also vascular, 
or FiBRO-VASCULAR, BUNDLES, which ruu lengthwise of the 
stem, and have the same general structure, and the same 
function of conduction for water and food, as in the leaves. 
Commonly they are arranged in a ring, in which case they 
enclose a pith, of loose open texture, often glistening-white 
from included air. The pith is especially the storage part of 




Fig. 70. — The tissues of a typical 
herbaceous stem, of the Stock, in cross 
section ; X 55. The cambium is repre- 
sented by the heavier double line through 
the fibro-vascular bundles, which are 
seven in number. The collenchyma is not 
marked. (From Scott, Structural Botany.) 



Ch. IV, 2] STRUCTURE OF STEMS 



117 





Fig. 71. 

Fig. 71. — Generalized sectional drawings, based on the Maple, to show 
the tissues of a typical stem. Explanation in the text. Secondary growth 
begins in the lower of the longitudinal sections. The cambium is left white. 

Fig. 72. — Companion series to Fig. 71, based on a Palm as the other 
type of stem. 

(From Sargent, Plants and their Uses.) 



118 



A TEXTBOOK OF BOTANY 



[Ch. IV, 2 



young stems, though other tissues share in that function. All 
of these features are shown with particular clearness in Fig. 71. 

In sections taken well back of 
the tip, two other tissues appear. 
One is a mere hne extending 
right through the fibro-vascular 
bundles, and from one to another 
(Figs. 70, 71), uniting them into 
one ring, or (since they run 
lengthwise) one sheath. This 
is the important cambium, or 
growth tissue, which later builds 
new tissues on both its outer and 
inner surfaces. The other is a 
band of whitish-gHstening tissue 
just beneath the epidermis, 
called coLLENCHYMA. It has a 
firm elastic texture, and aids 
the young stem to support the 
strains imposed by the presence 
of the leaves. Its position close 
to the outside is typical of the 
strengthening tissues of stems, 
which are developed upon the 
principle of the hollow column 
or tube. This principle is 
known to engineers as that 
which provides the greatest re- 
sy!::^.Z~JXZ7::^t ^^^tance to kteral strains with 

the least expenditure of ma- 
terial, on which account it is 
used by them in many construc- 
tions, — most f amihar perhaps in architectural columns and 
bicycle frames. 

The fibro-vascular bundles (or veins) of the stem extend 
downward all the way to the tips of the roots, and upward 




ing, in the young stem of Clematis 
viticella. (After Nageli, from 
Strasburger.) 



Ch. IV, 2] 



STRUCTURE OF STEMS 



119 



into the buds. Just below the leaves some of the bundles 
fork, and each sends one branch, called a leaf trace, into 
a leaf, and the second up the stem, as indicated in the 
typical example here pictured (Fig. 73), and as can be 
seen directly in a translucent stem hke that of the Balsam. 
This branching and rejoining of the bundles produces the 
node, which is thus explained, while thereby the bundles 
are united into one great 
cylindrical network or 
system. In this cylinder 
the turning of bundles out 
into the leaves results in 
gaps just above them; and 
around these gaps the new 
developing fibro-vascular 
cyhnders of the axillary 
buds establish their connec- 
tion with the main cyhnder 
(Fig. 71). 

While in most herbaceous 
stems the bundles are so 
arranged as to form a ring 
when seen in cross section, 
in others they are scattered irregularly, as illustrated here- 
with (Fig. 74). In such cases the bundles anastomose in the 
stems and extend out into the leaves in a manner differing in 
details, but not in principle, from the methods just described 
(Fig. 72). Thus the bundles collectively constitute a con- 
tinuous conducting system for water and food throughout 
the plant. 

The tissues above considered are all formed in the buds, 
and belong to the primary growth of the plant. Later the 
cambium, and other growth layers, add new tissues, which 
thus belong to the secondary growth. 

Woody stems develop from an herbaceous condition, 
through stages easily observed in the twigs of our common 




Fig. 74. — Stem of Corn, in cross 
section ; X 5. (Drawn from a photo- 
micrograph by Stevens.) 



120 



A TEXTBOOK OF BOTANY 



[Ch. IV, 2 



' 0' 



Fig. 75. — Winter 
twig of Horse Chest- 
nut; X }. 



trees during the first winter (Fig. 75). 
The leaves are now gone, not to reappear 
on this part of the stem; but the leaf- 
scars remain, marked by a Hghter colored 
corky layer, in which can be seen the 
severed ends of the veins. Each scar of 
course stands at a node, sometimes plain, 
but often not, just above which is the now 
prominent axillary bud, while a larger 
terminal bud ends the twig. The thin 
epidermis has been replaced by a layer of 
gray-brown waterproof cork, scattered over 
which are the lighter colored warty ex- 
crescences called LENTICELS. 

The leaf-scars and lenticels need special 
comment. Leaves fall from trees because 
of the formation of a special absciss-layer 
of tissue which develops across the base of 
the leaf in late summer (Fig. 76). Gradu- 
ally this layer closes the free communica- 
tion between stem and leaf, though mean- 
time the valuable materials of the leaf are 
mostly transferred to the stem. Then 




Fig. 76. — Vertical section through a 
twig and petiole of Poplar, showing the 
absciss-layer, a.l. (From F. Darwin, 
Elements of Botany.) 



Ch. IV, 2] 



STRUCTURE OF STEMS 



121 




follows the waning vitality, cessation of chlorophyll forma- 
tion, appearance of autumn coloration, and finally, by a 
weakening of the 
walls of the absciss 
layer, the fall of the 
leaf itself, the absciss 
layer becoming the 
corky and waterproof 
leaf -scar. The lenti- 
cels are physiologi- 
cally important 

Q+rnp+nrP<5 for fhpv ^^^- ^7. — A typical leiiticel, of Samhucus nigra, 
Structures, lOr tney in section ; magnified. (From Haberlandt.) 

replace the stomata 

(which disappear of course with the epidermis), as avenues 
of gas exchange between the interior of the stems and the 
external atmosphere. This exchange is no longer needed 
for photosynthesis, which ceases as cork develops, but is nec- 
essary for the respiration of the living tissues within, as will 

later be shown. The lenti- 
cels are places where a loose 
tissue with inter-cellular 
spaces is formed instead of 
the impervious cork; and 
this tissue by its growth 
partially closes them in 
winter and forces them open 
the next spring (Fig. 77). 

The tissues of these tran- 
sitional stems show very 
clearly in cross section (Fig. 
Fig. 78. — a cross section through 78). Their most striking 
a winter twig of Tulip Tree; X 10. feature is the sharp division 

The lighter continuous line is the cam- 
bium, and the medullary rays are dis- between bark and WOOd at 

*^^^*- the cambium. The parts of 

the bundles inside the cambium have grown greatly, and 
show clearly the characteristic forms and texture, while the 




122 



A TEXTBOOK OF BOTANY 



[Ch. IV, 2 



tissue between them is reduced to fine radiating lines, which 
henceforth are called the medullary rays. These woody 
parts of the bundles, called xylem, contain the ducts, and 
conduct water through the stems. Inside the cylinder of 
bundles is the very distinct pith. In the pith is much 
starch, which is food for the next season's growth, though 
it occurs also in medullary rays and bark, often in strikingly 
symmetrical patterns when set forth in blue by the iodine 
t^st. Outside of the cambium can be seen, though only 




Fig. 79. — Stages in the healing of a pruned stem, cl indicates callus, a 
tissue which precedes the overgrowing bark. (After Curtis, from Duggar, 
Plant Physiology.) 

imperfectly by hand lenses, the outer, or phloem, parts of 
the bundles, which contain the sieve tubes and conduct food 
through the stem. The remainder of the bark is composed 
mostly of the former chlorenchyma, now fast losing its chloro- 
phyll, and known henceforth by its morphological name of 
CORTEX, while the temporary collenchyma and epidermis are 
being replaced by layers of waterproof cork, made by a cork 
cambium, and pierced here and there by the lenticels. All 
of these features can be traced very easily in nearly all twigs. 
The tissues of plants have a remarkable power of healing 
injuries which befall them. Any break in the soft tissues is 
healed partially within a few hours, and completely within 



Ch. IV, 2] 



STRUCTURE OF STEMS 



123 



a few days, by formation of cork layers, often manifest by 
their brown color. Where an injury includes the wood, as in 
case of broken branches or the pruning of large trees, the 
wood itself does not heal, but the neighboring bark, and also 
the cambium, gradually overgrows it. In time the cambium 
reestabhshes itself over the injury and then continues to make 
wood as before (Fig. 79). This power of healing injuries 
has high value for plants, since their epidermis and cork 
form not only a protection against dryness, but serve also 




Cross section through bark and wood of an old Elm tree, 
showing abscission of the bark ; X f . 



as their first Hne of defense against the entrance of injurious 
parasites, which are ever ready to enter any break in the 
tissues of the stem. 

With increasing age several new features appear in woody 
stems. Sections then show that the outer part of the bark, 
which is dead, is cut off from the interior living part by 
layers of cork, which form anew each year, much as the 
absciss layers form in the bases of leaves (Fig. 80). As in case 
of leaves, also, the valuable materials in the outer bark are 
previously removed to the stem. This dead bark becomes 
vertically cracked by the pressure of the expanding wood 
within, and the resultant fissures replace the lost lenticels 
as avenues of gas exchange between the interior of the stem 



124 A TEXTBOOK OF BOTANY [Ch. IV, 2 

and the atmosphere. Further, the outer dead bark steadily 
weathers and falls away, either somewhat evenly as in Beech, 
or else in great flakes cut off by the cork layers as in Elm, 
Hickory, Oak, or in remarkably smooth layers as in Birch. 
The inner living part of the bark consists of soft, continuously 
growing tissue, together with the phloem parts of the bundles. 
In the older stems, both wood and bark are greatly in- 
creased in thickness as result of the activity of the cambium, 
the growth layer of the stem, which continuously forms 
new wood on its inner and new bark on its outer face. 
This process goes on indefinitely, making the woody trunk 
grow steadily in thickness. The bark, however, is simul- 
taneously weathering and peeHng away on the outside, and 
there comes a time when the rate of this weathering just 
about keeps pace with the additions within, thus holding 
the bark thenceforth of nearly constant thickness, though 
in constant renewal. In the wood only a few outer layers 
forming the sap wood, distinguishable by the Ught color, are 
ahve, while the heart wood, usually much darker colored, 
is all dead ; and the heart may even decay and vanish com- 
pletely, leaving a mere shell of sap-wood, which, however, 
suffices, on the hollow column principle, to support the tree. 
The cambium forms the annual rings, one each year 
(Fig. 80). It is easy to see that the appearance of the 
rings is due simply to the contrast between the loose open 
texture of the wood formed in spring, when large quantities 
of water, carrying with it stored food, are needed for the new 
growth of the herbaceous parts, and the close compact growth 
of the autumn, when less water, and no such food, are re- 
quired. It is these annual rings which, when cut lengthwise, 
give the distinctive, attractive ^' grain" to cabinet woods. 
Not only do annual rings appear in the wood, but they also 
occur in the bark, though here they are difficult to see 
(compare Fig. 87), because the tissues are soft, and soon 
crushed, and later cut off by the cork layers. Since they 
are formed by the cambium, the older layers of bark 



Ch. IV, 2] 



STRUCTURE OF STEMS 



125 



are outside, in reverse of the condition in the wood, as 
shown in principle by our diagram (Fig. 81). The third 
new feature consists in the secondary medullary rays 
(Fig. 82). They form in the ever-broadening fibro-vascular 
bundles, which thereby are kept divided to nearly their 
original -^vidth. It is hardly correct, however, to speak 





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Fig. 81. — Diagiinn of a cross section of a generalized stem, to illustrate 
the interrelations of fibro-vascular bundles, pith, medullary rays, both 
primary and secondary, cambium (black), cortex, and cork. Annual rings 
in bark and wood of identical age are identically shaded. The extension of 
the rings across the medullary rays is not shown, though it is usually plain 
in the wood while obscure or absent in the bark. 



any longer of separate fibro-vascular bundles, since their 
identity has long since been lost in that of the general woody 
mass and the bark. 

The medullary rays are an important, and sometimes a 
conspicuous feature of the wood. Beginning as plates of 
tissue between the originally separate bundles, they are 
later developed and multiplied in number as a persistent part 
of the wood, in which they serve as avenues of communica- 



126 



A TEXTBOOK OF BOTANY 



[Ch. IV, 2 



tion between the inner and outer layers. They do not run 
far, as a rule, up and down the stem (Fig. 82), no farther 
than the distance between the successive f orkings of the fibro- 
vascular bundles in the original bundle cylinder (page 119). 
They are more prominent in some woods than others, and are 
especially striking in Oak, where they form the prominent 

radial lines so plain 
on cross sections, and 
the irregular shining 
plates for which Oak 
is ''quartered", that 
is, cut longitudinally 
in a way to display 
them. The Oak has 
also ducts so large as 
to be clearly visible 
to the naked eye, — 
whence its conspicu- 
ous grain. 

Stems exhibiting 
clear distinction of 
bark, wood, and 
pith, having cam- 
bium, annual rings 
and medullary rays, 
and increasing in- 
definitely in thick- 
ness by secondary 
growth, represent the most highly developed type, which in- 
cludes all of our common trees and shrubs. Since they grow 
by additions of layers to the wood, they are called exogenous. 
The other prominent type has none of the above-mentioned 
features, but remains permanently in a primary growth con- 
dition with the bundles scattered irregularly throughout the 
stem (Figs. 72, 74). In the belief, since found erroneous, that 
such stems grow by addition of new bundles inside of the 




Fig. 82. — A 4-year-old stem of Pinus sylves- 
tris, with bark partially removed at the cam- 
bium ; magnified. It shows clearly the medullary 
rays, primary and secondary, and the annual 
rings, containing resin canals. (From Stras- 
burger.) 



Ch. IV, 2] 



STRUCTURE OF STEMS 



127 



older, they were named endogenous, and the name remains 
in use. This type is characteristic of Grasses, LiHes, Palms, 




Fig. 83. -^ Typical exogenous and endogenous stems, in cross section, of 
Red Pine and a Palm ; X f. (Drawn from photographs.) 

and in fact of all plants in the great natural group of the 
Monocotyledons, where it is associated with parallel-veined 
leaves, and sparse branching. 
The contrast between the two 
types appears very clearly in our 
picture (Fig. 83). The typical 
endogenous type does not permit 
an indefinite increase in diameter, 
for, after the fibro-vascular 
bundles first laid down have in- 
creased to their full size, the stem 
no longer enlarges in diameter, 
but only in height, whereby en- 
dogenous plants are rendered 
extremely slender and graceful, as 
Palms and Bamboos illustrate. 
The great heights maintained by 
such stems with slender diameters 
rest partly on the yielding elasticity permitted by the long 
curving courses of their separate fibro-vascular bundles 




Fig. 84. — The Dragon Tree, 
Dracoena Draco, of the Canary 
Islands, an endogenous plant 
which grows indefinitely in diam- 
eter. (From Balfour.) 



128 A TEXTBOOK OF BOTANY [Ch. IV, 3 

(Fig. 72), and partly on the perfection to which the hollow- 
column principle is carried in their construction, as witness 
the Bamboo. Upon the latter feature they depend far more 
than do exogenous plants, which find ample support in their 
massive solid trunks. Some Monocotyledons, however, do 
exhibit increase in diameter, for the outer layers of their 
stems develop a cambium-like tissue which continues to 
form new scattered bundles as long as the plant hves. It is 
thus that the great Dragon Tree, though endogenous, can 
attain to so great a diameter and age (Fig. 84). In all endog- 
enous plants, the seeming bark is nothing other than the 
compact outer tissues, darkened more or less by action of 
the weather, of which the effects penetrate to some depth. 

Striking though the difference appears between the 
exogenous and endogenous types of stems, they perform the 
same functions with apparently equal efficiency. The 
differences between them are therefore not functional, but 
depend rather upon their relationships within two dif- 
ferent and ancient lines of evolutionary descent. Did we 
not know this fact, we might seek long for a functional ex- 
planation of differences the significance of which Hes only in 
heredity. 

3. The Cellular Anatomy of Stems 

From the tissues of stems, which can readily be recognized 
by aid of a hand lens, we turn naturally to consider the 
constituent cells, making use of the microscope. 

One of the best stems for such study, because of its ex- 
ceptionally clear definition of the parts, is that of the Dutch- 
man's Pipe {Aristolochia Sipho), a common vine. Sections 
through the terminal bud, or very close thereto, show only 
the closely packed, squarish, protoplasm-filled cells which 
one soon learns to associate with the embryonic stage of 
growth (compare Figs. 92, 162). Such embryonic tissue is 
always called meristem, whether in buds, growing tips of 
roots, cambium, or elsewhere. A little behind the bud 



Ch. IV, 3] 



ANATOMY OF STEMS 



129 



Fig. 85. — The cellular anatomy, in corresponding cross and longitudinal 
sections, of a young stem of Aristolochia Sipho, a twining vine ; X 50. 
In the center, the pith ; next three fibro-vas- 
cular bundles, showing the xylem with 
ducts, the rectangular nucleate cells of 
the cambium, and the phloem, 
marked by sieve-plates and nu- 
cleate cells ; next the sclerenchyma 
ring, with the starch sheath 
just outside ; then the cor- 
tex, consisting of chlo- 
renchyraa and coUen- 
chyma ; and finally 
the epidermis. 




130 A TEXTBOOK OF BOTANY [Ch. IV, 3 

the cells are found well differentiated, as our picture illus- 
trates (Fig. 85). Outside is the single layer of the epidermal 
cells, with occasional stomata, not essentially different from 
those in leaves. Just beneath lies the zone of coUenchyma 
cells, of which the thickened angles, elongated forms, and 
composition from elastic cellulose explain their function as 
the first strengthening tissue of the flexible and elongating 
stems. Next comes the chlorenchyma, like that of the leaves, 
though with scantier chlorophyll. Its innermost layer con- 
tains starch, and constitutes the staech sheath, of which 
the function is disputed, some investigators assigning it a con- 
ducting function for carbohydrates, while others consider 
it a geotropic-perception sheath, as will later be explained. 
Next inside comes a very prominent zone of angular, thick- 
walled, Ught-colored, greatly elongated empty cells, found by 
tests to be hard and stiff. These are sclekenchyma cells, 
the characteristic strengthening cells of plants, found in 
diverse situations, and here evidently giving special support 
to the young stems of this vine, which stand out laterally 
before twining around a support. 

Inside the sclerenchyma ring can be seen the fibro-vascular 
bundles, which here present an unusually distinct structure. 
Each bundle is ovate in cross section, with the point towards 
the center, and shows three parts. The xylem, or wood, 
inside of the cambium, contains the large, somewhat angled, 
thick-walled ducts, lacking protoplasm but variously marked 
by spirals, pits, and the Hke. They are formed by the 
union of many cells of which the intermediate walls have 
been absorbed. Intermingled therewith are smaller cells, 
partly wood fibers and partly wood parenchyma, having 
minor functions in connection with the conduction and 
storage of carbohydrate foods. The characteristics of the 
xylem are further well illustrated in the diagrammatic 
figure 86. 

The PHLOEM, or bast, is composed of small thin-walled 
elongated protoplasm-containing cells lying outside of, and 



Ch. IV, 3] 



ANATOMY OF STEMS 



131 



matching, the xylem strands. The larger cells are crossed 
here and there by the perforated plates which show them 
to be sieve-tubes; and they are the protein-conducting 
parts of the bundles, precisely as in leaves (page 31). Inter- 
mingled with the sieve-tubes are other slender cells, com- 
panion CELLS, which have something to do with the function 
of the sieve-tubes, and bast parenchyma cells, in which 




Fig. 86. — Generalized drawing of an exogenous stem, to show the 
typical anatomy of the cellular elements ; highly magnified. From left to 
right, the cork, the cortical parenchyma, starch sheath, bast fibers, phloem 
parenchyma, sieve-tube, cambium, ducts with xylem parenchyma, and pith. 
(From Kerner.) 

carbohydrates are conducted, and which, therefore, along 
with the wood parenchyma, take the place in stems of the 
conducting bundle sheath of leaves (page 30). Often the 
phloem contains in addition very long and thick-walled but 
flexible fibers, called bast fibers (Fig. 86), which give stiff- 
ness to the stem when sclerenchyma is wanting. It is these 
bast fibers which in the flax plant yield us our linen, and 
in some trees provide tough strands utilized by savage tribes 



/ 



132 A TEXTBOOK OF BOTANY [Ch. IV, 3 

for cords, and even a lace-like material serviceable for 
fabrics. 

The cambium of the fibro-vascular bundles Ues between 
xylem and phloem. It consists of several rows of compact, 
rectangular, thin-walled, elongated, protoplasm-filled cells, 
having the meristematic aspect which is always associated 
with growth (compare Fig. 92). In older stems, lines of cells 
in the tissue between the bundles (the beginning of the medul- 
lary rays), become converted into cambium, continuous with 
that in the bundles, and thus the cambium ring is completed 
around the stem. It then forms a perfectly continuous cyHn- 
drical sheath between wood and bark on trunks, branches, 
and roots, and it merges imperceptibly into the meristem of 
the buds and root tips, which are thus brought into a single 
continuous growth system ; but it does not occur in leaves. 
Being a growth tissue, and therefore thin-walled, it is easily 
torn, which explains why bark is so readily removable from 
wood, especially in spring when the cambium is most active 
and tender. Indeed, at this season the cambium can be 
stripped in long gelatinous sheets from the wood of some 
trees, notably white pines. In its growth it divides contin- 
uously in its own plane, the cells on its inner face becoming 
new xylem elements, and those on its outer face new phloem 
elements, while the intermediate cells remain cambium. 
In this manner it builds also the medullary rays of both wood 
and bark. 

Inside the ring of bundles he the cells of the pith, exhibit- 
ing the large sizes and rounded forms associated with storage, 
whether of food or water. Among them appear very clearly 
the intercellular air-spaces, which can likewise be traced in 
other parts of the stem, although it has not been possible to 
show them in our small scale drawing (Fig. 85). In reahty 
they are parts of a continuous intercellular aeration system 
which extends from the pith along the medullary rays and 
through the outer tissues to the lenticels and the exterior 
air. In the pith, as in other parts, can frequently be seen 



Ch. IV, 3] 



ANATOMY OF STEMS 



133 




crystals, which have the 
significance already ex- 
plained for those of the 
leaf (page 111). 

Woody stems exhibit 
their cell structure very 
clearly in sections (Fig. 87). 
In the bark can be seen the 
flat, continuous, brownish 
cells of the cork, made by 
a special cork-cambium just 
beneath them. The first 
cork is usually formed just 
beneath the epidermis, 
w^hich it replaces ; but later 
the cork-cambium forms 
anew each year at some 
distance from the surface, 
thus building the layers of 
cork which cut off the areas 
of bark (Fig. 80). The 
wood shows clearly the 
various cells of the xylem 
and medullary rays (Fig. 
87), as likewise the cellular 
construction of the annual 
rings, with the contrast 
between the loose open cells 
of the spring wood and the 
compact growth of the pre- 
ceding autumn. 

Fig. 87. — A segment, in cross section, 
of a stem of Linden. From without inward are epidermis (here unusu- 
ally persistent) ; cork ; cortex (the starch sheath not shown) ; phloem, con- 
sisting of alternating layers of bast fibers (lighter) with sieve and paren- 
chyma elements ; cambium ; xylem, showing three annual rings, with large 
ducts, wood cells, and (on the sides) medullary rays ; and pith. The view 
shows one complete fibro-vascular bundle, three years old. (Drawn from 
a wall-chart by L. Kny.) 



134 



A TEXTBOOK OF BOTANY 



[Ch. TV, 3 



Some stems present special cellular features, of which the 
most striking is tlu^ latex system. This latex, the milky 
juice of plants which contains so many diverse substances 
of uncertain significance (page 108), is found mostly in long, 
slender, closed tubes branching irregularly through the 

softer tissues, and even the 
.wood of stems, leaves and 
other parts (Fig. 88). 

The Aristolochia, and 
other stems just men- 
tioned, are all exogenous. 
The endogenous type pre- 
sents some, though no 
great, cellular differences. 
Thus, as exempHfied in the 
Corn (Fig. 89), the bundles, 
of course lacking cambium, 
present each a very distinct 
strand of thin-walled, regu- 
larly-arranged phloem, 
alongside of which is the 
strand of xylem, distin- 
guished by very large ducts 
and commonly a great air 
passage; while the apparent bundle-sheath encircling the 
bundle has been found to develop by extension from the 
xylem. In such stems there is no distinction of pith, 
medullary rays, and cortex; but all are merged together 
in one pith-hke general ground tissue (Fig. 74) . 

In the foregoing description of the structure of stems, we 
have considered only one type of fibro-vascular bundle, — 
the kind having parallel strands of phloem and xylem. 
Many other types and sub-types, however, occur, as well as 
many special forms of arrangement of the bundles within 
the cyhnders and in relation to the leaves. It has recently 
been found that these morphological features of stem 




Fig. 88. — The latex system in Lac- 
tuca virosa, in section ; X 180. (From 
Kerner.) 



Ch. IV, 4] DEVELOPMENT OF STEMS 135 

anatomy are very stable in heredity, thus making them good 
guides to the evolutionary history and present relationships 
of plants. This important phase of investigation is now in 




Fig. 89. — A single bundle in Corn (one from those shown in Fig. 74), 
in cross section ; X 130. s points to the strand of phloem ; m and sp are 
ducts which, with the intermediate cells, form the xylem ; Z is an air space 
containing a ring, a, from a duct ; vg indicates the sheath around the bundle. 
(From Strasburger.) 

active and successful development, but is somewhat too 
special in method for further consideration in an introduc- 
tory course. 

4. The Development of Stems and Leaves from Buds 

Stems and leaves originate together in buds, though it is 
more exact to say that the embryonic condition of a stem 
with its leaves constitutes a bud. 

Most famihar of all are the winter buds of trees, in which 
the bud proper is enwrapped within corky brown scales, 
often with accessory linings of hairs or coatings of resin, as 



136 



A TEXTBOOK OF BOTANY 



[Ch. IV, 4 



the Horse Chestnut, for instance, well illustrates (Fig. 75). 
The scales, which are modified leaves of the preceding 
year's growth, though prominent, are not an essential' part 
of a bud, having only the temporary function of protecting 
it over winter, after which they fall. The scales are lacking 
from all summer buds and some winter ones. The really 
essential feature of a bud is the embryonic stem composed 
of meristem or active growth tissue, together with the 




Fig. 90. 



A head of Lettuce, in section, illustrative of bud structure : 
X h (From Bailey.) 



nascent leaves which grow out laterally therefrom. The 
lower and older leaves of a bud commonly overlap and cover 
the upper and younger for a time, but later open out to begin 
their work; and in herbaceous plants a perfect gradation 
is often apparent between the nascent leaves in the bud 
and the full grown leaves of the stem. 

Buds are of all sizes, from too small to be seen without the 
aid of a lens, up to several inches in diameter, as in Palms. 
A Cabbage or head of Lettuce is essentially a gigantic bud, 
and offers a particularly favorable illustration of the essentials 
of bud structure ; for a section made lengthwise through its 



Ch. IV, 4] 



DEVELOPMENT OF STEMS 



137 



center shows very clearly the characteristic tapering stem, 
with the series of leaves in all stages of development 
(Fig. 90). 

Within the buds the leaves are arranged in various ways, 
either overlapping, or each folded by itself on its midrib, or 
inroUed from margins or tip. The arrange- 
ments are called collectively vernation, and 
have importance in descriptive taxonomy. 

The most prominent, and commonly the 
largest, buds are those which are terminal 
on the main stems and branches, and which 
continue the stem growth. More abundant 
are the axillary buds which develop in the 
upper angle between leaf and stem, and are 
nearly as numerous as the leaves themselves, 
at least in exogenous plants. The functional 
reason for the usual occurrence of buds in this 
position is found, no doubt, partly in the near- 
ness to the source of food indispensable for 
their development, and partly in the favorable 
structural opportunity to make connection 
with the main stem in the gap left in the 
fibro-vascular cyhnder above the leaf base 
(page 119) . In a few plants, of which Tatarean 
Honeysuckle and Red Maple are examples 
(Fig. 91), more than one bud occurs in each 
axil, either side by side, or one above another, Fig. 91, — A 
the extra buds being called accessory. Maple, showing 
Finally, while in many plants no buds other accessory buds ; 
than terminal or axillary occur, in others they Gray.) 
develop in almost any position, especially at 
some place of injury; and these so-called adventitious 
BUDS produce the branches in irregular positions, as often 
seen in Willows and some other woody plants. Not all 
irregular branching, however, results from adventitious 
buds, for axillary buds often remain latent for years, becom- 



138 



A TEXTBOOK OF BOTANY 



[Ch. IV, 4 



ing deeply buried under the bark, and yet finally give origin 
to branches. 

Where the terminal bud is notably prominent, as in Horse 
Chestnut (Fig. 75), the axillary buds are largest just below 
it, and progressively smaller farther back. Such a terminal 
bud unfolds its flowers, stem, and foHage very quickly and 
makes no more growth in length that season, though the upper 
axillary buds may develop into branches. Such is definite 
ANNUAL GROWTH. Where the terminal bud is relatively small, 
as in Elm, it continues to grow and produce new leaves and 
axillary buds all summer, and new branching takes place from 
the lower new buds. Such is indefinite annual growth. 

The occurrence of a bud in the axil of every leaf gives the 
plant a great surplus, of which few ever develop into branches, 
though all are capable of so doing. Generally speaking, 
those nearer the outer ends of the branches, and therefore 

nearest the Ught 
and free space, 
are the ones 
which develop, 
though if the ter- 
minal, or outer 
axillary, buds be- 
come destroyed, 
whether by frost, 
insect ravage, or 
experimental de- 
sign, the next 
lower develop in 
their places. 
Since all are 
capable of de- 
velopment, it is evident that some factor controls them 
collectively, either inhibiting the development of some or 
stimulating that of others, — a subject to which we shall 
later return under Growth. 




Fig. 92. — A bud, of unusually elongated form, 
of a water weed, Elodea canadensis, in exterior view 
and section, showing the development of leaves ; 
X 150. (After L. Kny.) 



Ch. IV, 5] ARRANGEMENTS OF LEAVES 



139 



The mode of formation of stem and leaves within buds 
is revealed by longitudinal sections. In an illustrative case 
(Fig. 92), one can see very clearly the blunt conical end of 
the stem, composed of small tightly-packed cells in process 
of formation through new cell-divisions, while backwards 
the cells are evidently beginning to elongate with the 
lengthening of the stem. The first visible trace of a leaf is 
found in the enlargement of a superficial cell, which soon 
divides ; the resultant cells again divide and, including both 
epidermis and cortex, gradually push out in a flat projection, 
— the leaf. These leaves enlarge steadily, thus making a 
perfect gradation from those just appearing to those fully 
formed. As they develop, the nodes on which they stand, 
at first close together, are carried apart by lengthening of 
the internodes, and the tissues gradually pass from the 
meristematic, or embryonic, to the differentiated or mature 
condition. In this process, however, the areas of tissue in 
the axils of the leaves remain meriste- 
matic, thus originating the axillary 
buds. 

5. The Arrangements of Leaves 
ON Stems 

Leaves develop upon stems not at 
haphazard, but in definite math- 
ematical order. This definiteness 
of arrangement, called scientifically 
PHYLLOTAXY, while sometimes obscure, 
is often strikingly manifest to the eye. 

When two leaves occur at a node, 
they are always opposite to one an- 
other, and each pair stands at right 
angles to the pairs above and below, 
thus forming four ranks on a vertical 
stem, as the Coleus of our gardens, 
and the Mint family in general, well 




Fig. 93. — The opposite 
arrangement of leaves, as 
illustrated by a museum 
model. 



140 



A TEXTBOOK OF BOTANY 



[Ch. IV, 5 



illustrate (Fig. 93). When the stem is not vertical, how- 
ever, the leaf blades swing around phototropically on their 
petioles until they face upward, in a 
sort of mosaic, towards the light 
(page 57), thus obscuring the op- 
posite arrangement, which, however, 
can always be seen where the petioles 
join the stems. It is important to 
remember that phyllotaxy is a matter 
of the place of origin of leaves upon 
stems, and has little to do with the 
positions which the leaf blades ulti- 
mately assume. In some kinds of 
plants, not two, but three, or more, 
leaves occur at each node, forming 
a WHORL, in 
which case the 
leaves com- 
monly cover 
the gaps be- 
tween those 
above and below, as occurs in many 
plants of the Lily family (Fig. 94), 
The arrangement is particularly plain 
in the relation of the petals to the 
sepals in most flowers. Often, how- 
ever, it is rendered imperfect by 
twisting of the stems, variation of 
number of leaves in the whorls, and 
other less obvious causes. 

When only one leaf occurs at each 
node, they fall collectively into a 
spiral, and the leaves are said to be 
ALTERNATE. In the simplest case 

the successive leaves stand one-half way around the circum- 
ference of the stem from those next above and below, thus 




Fig. 



94. — The whorled 
arrangement. 



\ 



K 



\ 



(^ 



Fig. 95. 

^ spiral 



- The alternate, 
arrangement. 



Ch. IV, 5] ARRANGEMENTS OF LEAVES 



141 



forming two vertical ranks and bringing the third leaf over 

the first (Fig. 95), as well manifest in Corn and other 

Grasses. In other cases, the leaves stand 

one-third of the circumference apart, 

forming an obvious spiral, bringing the 

leaves into three vertical ranks with a 

fourth over a first (Fig. 96), as in Sedges, 

which, correlatively, have triangular 

stems. The next of the arrangements 

actually found is that where the leaves 

stand two-fifths of the circumference 

apart (Fig. 97), in which case the spiral 

must turn twice around the stem before 

a leaf, the sixth, is reached over the first, 
five vertical ranks 
resulting. This is 
the commonest of 
the alternate ar- 
rangements. It is 
very clear in the 
Apple, and in many ment. 
common plants, 

though at times, in long stems, its regu- 
larity is disturbed by some twist of the 
stem. It underlies the prevalence of the 
number five in the plan of most flowers, as 
the one-third and one-half arrangements 
underhe the numbers three and four in 
others. The next arrangement is that 
of three-eighths (Fig. 98), found in the 
Holly. The next is that of five-thir- 
teenths (Fig. 99), found in the House- 
leek, which forms the familiar rosettes, 
while in Pine cones and other such struc- 
tures, arrangements of eight twenty-firsts, and even thirteen 

thirty-fourths and twenty-one fifty-fifths have been deter- 





FiG. 96. — The alter- 
nate, I spiral, arrange- 



FiG. 97. — The alter- 
nate, f spiral, arrange- 
ment. 



142 



A TEXTBOOK OF BOTANY 



[Ch. IV, 5 



mined 
gence 



. These fractions primarily express the angular diver- 
of the leaves from one another around the stem, but 
secondarily the numerator shows the 
number of turns made by the spiral in 
. reaching a leaf directly over any given 

-^r:^^ one, while the denominator expresses the 

number of leaves in such a complete turn. 
It is not only true that these fractions 
are actually found in phyllotaxy, but 
also a fact that they are the only ones 
which ordinarily occur, the exceptions 
being rare, and following an analogous 
plan. Furthermore, when a stem having 
one of these fractional systems becomes 
twisted, the leaves are always brought 
into the next system above or below. 
When, now, the fractions are arranged 
in sequence, — 




Fig. 98. — The al- 
ternate, I spiral, ar- 
rangement. 



13 
34 



tt 



some remarkable relations among them 
come out, — viz. in all cases after the first and second, the 
numerators and denominators are each the sum of the two 
preceding, while each numerator is the 
same as the denominator next before the 
preceding. This curiously related series, 
which as a mathematical abstraction is 
known from its discoverer as the Fibo- 
nacci series, finds actual physical expres- 
sion not only in phyllotaxy, but also in 
some other phenomena of nature. 

The significance of phyllotaxy has been 
diversely interpreted. Some botanists 
have explained it as adaptive, thinking 
it must give to clusters of leaves the best aggregate ex- 
posure to light. But such reasonableness as this theory may 




Fig. 99. — Rosette 
of Houseleek, showing 
the J J arrangement. 
(After Gray.) 



Ch. IV, 5] ARRANGEMENTS OF LExWES 



143 



seem to possess in case of the opposite arrangement and the 
lower fractions of the spirals vanishes in case of the higher 
systems, which are inappreciabl.y different in so far as leaf 
exposure is concerned; while, moreover, the ultimate ex- 
posure of leaf blades is determined chiefly by their own photo- 
tropic movements on their petioles, with little or no regard 
to the plan of their origin. Later studies, however, seem to 
show that phyllotaxy originates in the construction of buds, 
as an incidental re- 
sult of the order in 
which the nascent 
leaves develop in 
relation to one an- 
other upon the cone 
of embryonic stem 
tissue. This order 
of development, in 
turn, seems to be 
connected with con- 
ditions of mutual 
pressure of the form- 
ing leaves upon one ^^^--^ ; /4Ki/^;r^\ii 
another in buds of 

diffprPTit «;hnnp^ fhic; ^^^- 100-— A head of Sunflower in seed, 

umereni snapes, Lnis growing its symmetry, which is an expression of 

pressure manifesting phyllotaxy. (Drawn from a photograph in the 

•j. ir vprv rliffpr R^Vort of the New Jersey Experiment Station for 

^ 1911.) 

ently in slender 

buds, which mostly produce the opposite system and low 
fractions, as compared with the broad or flat buds, which 
chiefly produce the higher fractions. Apparently the leaves 
originate regularly and successively in the lines of least 
resistance in the differently shaped buds, — and the result 
is phyllotaxy. Herein we seem to have a particularly clear 
case of one of those purely structural factors which were 
earlier mentioned as having a part with adaptation and 
heredity in determining details of plant form. 




144 A TEXTBOOK OF BOTANY [Ch. IV, 6 

Since new buds, which give origin to new branches, are 
axillary to leaves, the branching of plants should correspond 
with their phyllotaxy. This, indeed, is true in principle, 
as shown by young twigs ; but as plants grow older the 
regularity of their branching becomes greatly disturbed by 
irregular shading and diverse natural accidents. Flowers 
always originate from axillary buds, and hence clusters of 
flowers also exhibit the plans of phyllotaxy. This becomes 
especially striking when flowers are condensed closely 
together in heads, as in the Composite family ; and thus is 
explained the wonderful phyllotactic symmetry of Dahlia 
flowers, and of the head of a Sunflower in ^'seed" (Fig. 100). 
Other structures which show such symmetry strikingly well 
are cones of various trees, plants of compact growth, like 
the Mamillaria of the Cactus family, and various rosette 
plants. In all of these cases the primary spiral is difficult 
to trace because of its condensation ; but incidentally there 
arise a number of secondary and tertiary spirals, and these 
it is which become so strikingly evident. 

6. The Transfer of Water and Food through Plants 

A secondary function of stems is the conduction of water 
from the roots to the leaves, and of food from the leaves to 
the roots. We now consider the method of these important 
processes. 

In the lower plants, the Algae, Fungi, and Bryophytes, 
composed altogether of parenchyma cells without any, or 
with only a rudimentary, system of veins, both food and water 
are passed directly from one cell to another. The process is 
a slow one, and in land plants prevents any great develop- 
ment of size, as the very low growth of all Bryophytes, or 
Moss plants, exemphfies. The higher plants, however, both 
Flowering plants and Ferns, have developed veins, or vas- 
cular bundles, which permit the comparatively rapid trans- 
fer of both water and food through long spaces of stem, thus 
rendering possible the growth of those plants to tall trees. 



Ch. IV, 6] TRANSFER THROUGH PLANTS 145 

Of the two currents in the fibro- vascular bundles, the water 
current is by far the more voluminous, because of the great 
demands of transpiration. Some of the largest trees require 
each day Hterally tons of water, which must be raised one, 
two, or three hundred feet, and in rare cases still higher, into 
the air. To raise a given amount of water to a given height 
requires the expenditure of a definite amount of energy, 
no matter whether done quietly by a tree, noisily by an 
engine, or laboriously by human effort. It has been calcu- 
lated that the amount of work done, and energy required, 
to raise the water used by a large tree during twenty-four 
hours is approximately the same as that expended by a 
person in carrying three hundred large pailfuls of water up a 
ten-foot flight of stairs, — that is, a pailful every two minutes 
through a ten-hour working day. It is the botanist's prob- 
lem to explain the source of the energy whereby such great 
quantities of water can be raised to such heights against 
gravitation in small tubes which entail a great deal of friction. 
In other words, what are the forces which impel the rise of 
the sap in trees ? 

The water in passing along ordinary stems moves chiefly 
in the xylem part, especially the ducts, of the fibro-vascular 
bundles. This can be proven by experiment, for if an her- 
baceous stem be cut and stood in water dyed with some 
obvious color, e.g. red ink, and then later, as the first traces 
thereof appear in the younger parts, the stem be sectioned at 
different heights, the ducts will be found filled with the red 
fluid, which is also diffusing outward to the neighboring 
tissues. In a tree the water runs only in the younger xylem, 
i.e. the outer and younger rings of the white sap wood, and 
the colored heart wood has no part in the process. Thus is 
explained the fact that many kinds of trees can lose their 
heart wood by decay without detriment to water conduc- 
tion, as also the famihar fact that in tapping Maple trees 
for their sap, it is useless to bore more than an inch or two 
into the wood. 



146 



A TEXTBOOK OF BOTANY [Ch. IV, 6 



The water-conducting vessels are of two sorts, — first, 
elongated single cells, called tracheids, and second, tubes, 
called DUCTS or trachea (Fig. 101), which are formed from 

many cells of which the 
intermediate walls have 
been absorbed. Trache- 
ids occur often intermin- 
gled with ducts; they 
form the ends of the 
xylem part of the veins 
in leaves, and they make 
up wholly the secondary 
growth of Pines and other 
coniferous woods (Figs. 
102-4). Ducts develop 
usually from a single row 
of cyHndrical cells by 
absorption of the inter- 
mediate walls ; but some- 
times many rows of cells 
are involved, in which 
case the duct becomes 
large and visible to the 
eye, as in Oak and some 
vines, the single-row type 
being usually invisible 
without a lens. Though 
tubular in structure, 
ducts are never unlim- 
ited in length ; many are 
not more than a few 
inches, few exceed a few feet, and the longest, which occur 
in some vines, are only a few yards in length. In all known 
cases, however, the ends of ducts and tracheids are in con- 
tact with others of like sort, and the intermediate walls are 
so constructed, with guarded thin areas, as to permit a ready 




Fig. 101. — Generalized drawings of 
typical tracheal elements ; highly magni- 
fied. From left to right, a fiber-tracheid ; 
pitted and spiral tracheids ; spiral and 
pitted ducts, which show end walls and 
remnants thereof. (From Strasburger.) 



Ch. IV, 6] TRANSFER THROUGH PLANTS 



147 




passage of water moving at the ordinary rate, while resisting 
any forcible rush of the water along the stem under suddenly 
developed pressures. Thus the tracheids and ducts form 
water-conducting systems of unlimited length, even though 
the length of the individual elements is restricted. These 
thin areas, however, exist not only at the ends of the ducts, 
but throughout their 
lengths, where some- 
times they appear as 
bordered ' pits in an 
otherwise thickened 
wall, as is very charac- 
teristic of the conifer- 
ous wood (Fig. 104) : 
or else as the meshes 
of a reticulation : or 
as thin parts between 
spiral or annular thick- 
enings (Fig. 101), — all 
of which distinctive 
arrangements represent 
different ways of com- 
bining a thickening of 
the walls with the 
presence of thin places through which water may move to 
other ducts or tracheids, or to neighboring tissues. The 
annular and spiral markings are usually found in ducts or 
tracheids of the primary growth, in which elongation is still 
in progress, while other kinds occur in the secondary growth, 
where elongation has ceased. Both tracheids and ducts, 
when mature, are without protoplasm, forming non-hving 
tubes. Their mechanism, as a water-conducting system, is 
shown in our diagrammatic figure 105. 

The all-important question as to the forces by which the 
water is lifted through the ducts has been answered by in- 
vestigators in several different ways. In earlier times it 



Fig. 102. — Cross section through the 
wood of Pine ; highly magnified. 

The cells are mostly tracheids with bor- 
dered pits, visible in the walls. Note the 
medullary rays and the abrupt transition 
from autumn to spring growth. In the 
autumn wood is a resin canal. The line 
in all of the walls is the middle lamella, i.e. 
a plate representing the wall first formed 
before thickened by additional layers, and 
somewhat different in chemical and physi- 
cal composition from the latter, (Reduced 
from Cavers, Practical Botany.) 



148 



A TEXTBOOK OF BOTANY 



[Ch. IV, 6 




was thought that the water ascends by capillarity, precisely 
as oil rises in a wick, or water in a towel ; but experiments 
have proven that water cannot thus rise in wood more than 
a few feet. Also it has been held that atmospheric pressure, 
by which water is raised in a pump to a height of thirty-two 
feet, would explain it, the greater height reached in trees 

being supposed to 
result from the fact 
that intermixed air 
makes the water in 
the ducts much 
lighter than with- 
out it ; but further 
study has shown 
that neither are the 
conditions in the 
plant suitable for 
the operation of 
atmospheric pres- 
sure, nor would it 
suffice in very tall 
trees. Also it has 
been argued that 
the water ascends 
in the walls of the xylem by a process of imbibition, due to 
the attraction of wood for water, in precisely the same way 
that water passes into wood across the grain; but experi- 
ments have shown conclusively that the water goes through 
the cavities, not the walls, of the ducts. Still later it was 
claimed that living cells, in wood parenchyma or medullary 
rays, which accompany the ducts, act as a means of pro- 
pulsion of the water upward, each living cell absorbing water 
from the upper end of one duct and forcing it into the lower 
end of one higher, somewhat on the analogy of tiny force 
pumps ; but experiments seem to have proven that the water 
still rises when all living cells are killed by poisons. Finally, 



Fig. 103. — Radial section {i.e. parallel with 
a medullary ray) of the Pine of Fig. 102. 

A medullary ray runs across the tracheids, 
with the younger and outer end, containing pro- 
toplasm, on the left. Outermost, on the left, are 
sieve cells showing sieve plates on the walls ; next 
is cambium ; then tracheids of the spring wood, 
showing the characteristic bordered pits ; then 
autumn wood containing a resin canal. Com- 
parison with Fig. 102 will show the construction 
of a bordered pit, across which extends a thin 
and flexible plate. (From Cavers.) 



Ch. IV, 6] TRANSFER THROUGH PLANTS 



149 




most recently of all, a new and striking explanation has been 
offered, with much experimental support, to the effect that 
the water rises by traction, i.e. is drawn up in long threads, 
as if soHd, by forces acting in the leaves. This matter needs 
somewhat fuller explanation. 

In the next chapter it will be shown that the forces of 
osmotic pressure, operating in the roots, draw water from 
the soil and give it a start up the 
stem ; also the same forces in the 
leaves draw water in the same 
way from the ducts into the leaf 
cells. Now it is found that the 
forces thus exerted by the leaf 
cells are amply powerful to lift 
the water to the tops of the tall- 
est trees if only the water in the 
ducts would hold together in 
threads. The new theorj^ main- 
tains that the water does thus 
hold together, as if in soHd 
threads, by virtue of its own 
internal cohesion, a property 
which is manifest in part in the 
surface-tension famihar to all 
students of physics. Everybody knows that a large water- 
drop hanging free from the under side of a glass plate can 
be lifted with the plate, and it seems clear that the water 
could be lifted in much larger masses if lengthened out to 
very thin threads, as it is, of course, in the ducts. The 
water thus pulled into the leaves by the osmotic power of 
the leaf cells is removed from those cells by the still greater 
power of evaporation (transpiration), the energy for which 
is supplied by the heat of the surroundings. Thus, on this 
theory, it is really the energy of evaporation which raises the 
water in tall trees. But while evaporation is the principal, 
it is not the only source of energy available, for obviously 



Fig. 104. — Tangential sec- 
tion {i.e. at right angles to a 
medullary ray) of the Pine of 
Figs. 102-3, but more highly 
magnified. 

The tracheids, with ; their 
bordered pits, are plain, as are 
the cut ends of the medullary 
rays, of which one contains a 
resin canal. (From Cavers.) 



150 



A TEXTBOOK OF BOTANY 



[Ch. IV, 6 



any power whicli will draw water into the cells will lift the 
water-colunnis in the duct^. This result follows from 

various chemical or 
physical processes in 
which water is ab- 
sorbed ; and such se- 
cretory actions are 
beheved to explain 
the lifting of the sap 
in the spring before 
the leaves are de- 
veloped. This ex- 
planation is not yet 
universally accepted, 
many botanists still 
holding that the liv- 
ing cells along the 
stem are the chief 
factor in the process. 
It must, of course, 
be true that the 
greater the height of 
a tree, the greater 
the difficulty of rais- 
ing a sufficient 
transpiration supply 
against the increas- 
ing hydrostatic re- 
sistance, and the fric- 
tion within the small 
ducts. Thus a Hmit 
is imposed to the 
height of trees, which 
potentially can grow 
but actually have heights approxi- 
Our ordinary deciduous trees, 




Fig, 105. — Plan of the stem as a con- 
ducting mechanism, arranged as in Figs. 11 
and 166, with similar signs for protoplasm, 
water, sugar, and proteins. On the left is the 
pith, and then, in order, two ducts, a sieve- 
tube, phloem parenchyma, cortex, and cork 
with lenticels. 

upwards indefinitely 
mately fixed for each kind. 



I 



Ch. IV, 6] TRANSFER THROUGH PLANTS 151 

after reaching their heights, tend to spread out laterally and 
become flat topped ; and the dead branches which occasion- 
ally occur above their green summits represent certain ones 
which were able to exceed the ordinary limit in an especially 
wet summer, but died in a dryer one. Trees which attain 
to the greatest heights are apparently such as have par- 
ticularly favorable structural relations to conduction or 
transpiration. 

It is well known that in spring the sap exudes readily from 
injuries in many kinds of plants. Thus Grape Vines if 
pruned too late in the season will ''bleed" very copiously, 
and large drops of sap often fall upon sidewalks from broken 
twigs or bark of shade trees ; and the flow of sap in the Maple 
in spring is a well known phenomenon. These are evi- 
dently cases in which the sap is forced out by osmotic pres- 
sure in the roots, complicated, however, by osmotic pres- 
sures in the stem, and by expansion and contraction, under 
varying temperatures, of the air which occurs in the stem. 
The internal pressure developed by such expansion explains 
why the flow is greatest on a warm day after a cold night. 

The transfer, or more technically, translocation, of food 
through the plant occurs only in solution in water. In 
general the food has three paths. First, it may pass directly 
from cell to cell through protoplasm and walls by the power 
of diffusion, later to be studied. This is the sole method in 
the lower and simpler plants, and is that by which the food is 
removed from the chlorenchyma cells of the leaf to the near- 
est veinlets (page 32). It is also the method by which the 
food spreads from the ends of the veins to the growing cells 
in all parts of the plant. Second, in woody plants in spring 
the food stored in the roots or lower part of the stem is trans- 
ferred into the ducts (along the medullary rays from the 
bark), where it is rapidly Hfted to the growing leaves with the 
rising water current. This explains the presence of sugar 
in the sap of Maple and other trees in spring, though later 
in the season the sap is nothing other than soil water. Third, 



152 



A TEXTBOOK OF BOTANY 



[Ch. IV, 6 




111 

Mjh f 
mm 



^■;\r.^i 



Fig. 106. — The sieve plate, on 
surface and in section, of a typical 
sieve-tube of Squash ; X 400. 
Note that the protoplasm extends 
without break through the open- 
ings in the plate. (From Stras- 
burger.) 



the translocation occurs along 
special cells and vessels of 
the veins. Thus the carbo- 
hydrates, as already explained 
(page 31), move along the vein 
sheath in the leaves, and along 
parenchyma cells, chiefly of 
the phloem but partly of the 
xylem, in older bundles. All 
such conducting cells are elon- 
gated, possess plain walls, and 
contain protoplasm. The pro- 
teins have their clear paths of 
conduction in the sieve-tubes, 
of which this is the distinctive 
function. The sieve-tubes are 
elongated, thin, and smooth-walled cells, arranged in Hnes, 
with their intermediate walls perforated (J'ig. 106) ; and they 
have always a lining of living pro- 
toplasm. Associated with them are 
certain companion cells, which have 
seemingly a part, though an un- 
known one, in their function. So 
far as known, the proteins move 
along the sieve-tubes solely by dif- 
fusion from the places of greater to 
places of lesser abundance. The 
extreme slowness of this method, 
however, suggests that the living 
protoplasmic linings of the sieve- 
tubes may act in some way to force 
the movement, though there is no 
evidence thereof. 

Thus it is true in general that 
the movement of water in stems is 
chiefly in the wood, and the move- 




FiG. 107. — The result of 
constriction by a label wire 
on a growing shrub. (From 
Bailey.) 



Ch. IV, 7] GROWTH OF PLANTS 153 

ment of food is in the bark. Accordingly, if the bark be 
constricted, the passage of food downward from the leaves 
is stopped, and its accumulation produces a swelling above 
the constricting object. This happens in botanical gardens 
where labels are attached by wire, or in shade trees which are 
strengthened by iron bands placed around them (Figs. 107, 
155). The swelling may, however, go so far as to make the 
bark over-arch the constriction and establish a new connec- 
tion beyond it, thus burying band or wire completely. 

7. The Growth of Stems and Other Plant Parts 

It happens that stems exhibit the principal phenomena 
of plant growth more clearly than any other parts. Ac- 
cordingly we may best consider that important subject 
at this place. 

The growth of the higher plants differs from that of the 
higher animals in this, that while animals develop a single 
set of organs which serve throughout life, plants exhibit a 
constant and indefinite repetition of a few primary organs, 
— leaves, stems, roots, flowers, fruits, and seeds. But the 
general mode of growth of an individual leaf, stem, root, 
or other part is much the same as that of an animal organ. 

The growth of each individual organ or part, whether of 
animal or plant, exhibits three stages, which often overlap, 
but commonly are more or less separate. They are, first, 
DEVELOPMENT, or formation in the embryonic state : second, 
ENLARGEMENT, or increase in size, and third, maturation, 
or ripening to full functional efficiency. The difference 
between the stages comes out very clearly in case of the 
spring vegetation. First, as everybody knows, the flowers 
and leaves which unfold in early spring were all formed or 
developed the preceding season, and existed over winter 
folded compactly away in buds, in which they can easily 
be found. The microscope shows that most of the cells of 
those parts are perfectly formed, though small and filled 
with food. Second, in the early spring the buds, absorbing 



154 A TEXTBOOK OF BOTANY [Ch. IV, 7 

water from the stem, swell and open, and the leaves and 
flowers push forth and rapidly enlarge to their full size, 
as the familiar Horse Chestnut so strikingly illustrates. 
The microscope shows that this expansion is chiefly effected 
by a great increase in the size of the previously-formed 
cells, of which the large cavities are now empty of food, but 
tensely filled with water. That this enlargement is really 
brought about by absorption of water is very well proven by 
the fact that many such greatly-enlarged parts if thoroughly 
dried in an oven are found to be no heavier than in the 
original buds. This is especially clear in embryos, which, 
often germinating to a size a dozen or more times that which 
they had in the seed, actually lose dry weight in the process. 
The functional reason for this rapid spread of a little dry 
substance upon a great deal of water is plain ; the plant mode 
of life requires a spread of green surface as early as possible. 
Thirds the enlargement nearing completion, the parts mature, 
acquiring the final details of their coloration, thickening, and 
other features connected with the more effective performance 
of their functions. In this state they continue until their 
function is completied, after which they die and vanish 
through decay. 

The development, or formation (as distinct from enlarge- 
ment), of new parts takes place in meristematic cells (Figs. 
92, 162), which divide more rapidly in some places and direc- 
tions than in others. These divisions are apparently effected 
by the cytoplasm of the cells, which is controlled in the pro- 
cess by the chromosomes of the nuclei, as will appear more 
fully in connection with reproduction. 

We turn now to enlargement, the most striking and familiar 
phase of growth. So rapidly does it proceed in the stems of 
some plants out of doors in spring that its progress is visible 
from day to day. It is said that the growth of Bamboo in 
the tropics is so rapid as to be directly perceptible at times to 
the unaided eye, reaching often to more than two feet within 
twenty-four hours. Ordinarily, however, growth proceeds 



Ch. IV, 7] 



GROWTH OF PLANTS 



155 



so slowly as to need the microscope (specially arranged hor- 
izontally for the purpose), or else some other magnifying 




Fig. 108. — An auxograph in operation ; X f . 

The thread from the tip of the plant passes over the smaller of two 
united wheels, and is kept just taut by the weight of the pen, which 
hangs on the end of the thread passing over the larger wheel. The pen 
rests against the cylinder, which is turned once an hour by a clockwork 
in the closed case. The growth spiral marked by the pen is exaggerated 
in the cut. 

device, to make it apparent. A very effective instrument 
for this purpose, called an auxograph (meaning ''growth 
writer"), is shown by our picture (Fig. 108). The arrange- 



156 



A TEXTBOOK OP BOTANY 



[Ch. IV, 7 



ment is such that the growth of the plant permits 
a magnifying wheel to turn, and thereby a pen to 
fall along a cylinder which revolves once an hour, 
and upon which, accordingly, the pen marks a spiral 
line crossing any vertical line once an hour. Thus 
is obtained, night and day without break, an auto- 
graphic record of the plant's growth, an example of 
which, precisely traced, is given herewith (Fig. 109). 
As shown by such an instrument, the growth of 
any individual part, such, for example, as the 
flower-stalk of some bulbous plant, exhibits always 
two striking features. First, as our record well 
shows, there are many marked fluctuations in the 
rate. Second, aside from the fluctuations, one can 
always see that the rate of growth, instead of being 
uniformly rapid from start to finish, exhibits a slow 
beginning, a rise up to a culmination where it is 
most rapid, and then a gradual fall away to cessa- 
tion as the part approaches maturity. This mode 
of enlargement, which apparently results inciden- 
tally from the way the cells expand, is called the 
GKAND PERIOD. It is apparently characteristic of 
the growth of all individual parts, viz. of single in- 
ternodes of stems, of leaves, flowers, fruits, and 
really (though not apparently) of roots. In struc- 
tures composed of many unit parts, however, as in 
a stem with a number of internodes, the grand 
periods of the parts often overlap, and thus yield 



Fig. 109. — The complete record, obtained by the auxo graph 
of Fig. 108, of the growth of a flower stalk of Grape Hyacinth, 
from its appearance above ground until the completion of blos- 
soming. It is reduced photographically, from the 8-times mag- 
nification on the record papers, to the scale of the actual growth. 
It is also inverted from the record papers and therefore stands 
in the true position of the growth. Each space on the vertical 
line marks one hour, and the heavy horizontal lines indicate 
noon of each day. The lines which run together in the greatly 
reduced cut are perfectly distinct in the original record. 



Ch. IV, 7] GROWTH OF PLANTS 157 

collectively a continuous record. The grand period is 
most familiarly manifest in the opening out of the spring 
vegetation, in which, after a slow expansion of the buds, the 
actual opening takes place very rapidly, and full size is soon 
reached. In general the spring vegetation opens out on the 
crests of the grand periods of the parts concerned. 

As to the minor fluctuations of the growth records, they 
are found by experiment to have precisely the meaning 
which one naturally ascribes to them, viz. they are con- 
nected with the weather. If careful comparison be made 
between rate of growth and the contemporaneous mete- 
orological conditions, the following general results become 
evident. 

1. Higher temperature promotes growth, and lower checks 
it. This fact is of course sufficiently famihar, for everybody 
knows that plants grow faster in warm weather and slower 
when cool. The reason thereof is this, — growth involves 
a number of physical and chemical processes, all of which 
are directly promoted by heat. However, so far as plants 
are concerned, there are limits to its favorable action, be- 
cause above ordinary temperatures heat begins to act in- 
juriously upon the protoplasmic constituents, especially 
the susceptible proteins, which easily coagulate. Each 
plant has a minimum temperature below which it does not 
grow at all, an optimum temperature at which it grows fast- 
est, and a maximum temperature beyond which it ceases to 
grow. The conventional constants for these cardinal points, 
for our common plants collectively, are 5° — 30° — 40° C. 
The matter is illustrated very graphically when plants are 
grown in a differential thermostat (Fig. 110), in which they 
can be made to plot their own curve, so to speak, of their 
growth in relation to temperature. By use of this instru- 
ment the three points may be determined for any given 
plant, and thus it is also shown that in general the points 
range higher in tropical plants, lower in the arctic kinds, and 
intermediate in those of temperate regions. Since the 



158 A TEXTBOOK OF BOTANY [Ch. IV, 7 

cardinal points are ingrained in the protoplasm of particular 
species, it is clear why tropical plants do not thrive when 
taken abruptly to the arctics, and vice versa. The points, 
however, are alterable somewhat with time, as manifest in 
acclimatization. 

2. Humidity in the atmosphere (distinct altogether from 
the water supply at the roots) promotes growth, while 
dryness checks it. Everybody knows that plants grow best, 
on the whole, during close, "muggy" days, the kind most 
uncomfortable to us; and such days are called "growing 
days" by some farmers. The reason thereof is chiefly this, 




Fig. 110. — Effect of different temperatures upon Oats grown in a dif- 
ferential thermostat, an instrument evenly heated from the right side and 
cooled from the left ; X j-?,- 

that the enlargement of young parts is forced by the in- 
ternal pressure of water (the osinotic pressure explained in 
the next chapter) ; this internal pressure is lessened by re- 
moval of much water through high transpiration caused by 
dry air, but is maintained with low transpiration accompany- 
ing humidity of the atmosphere. Other things being equal, 
the growth of vegetation is more rapid in regions of moist 
climate, and slower in dry ones, as forests and deserts respec- 
tively illustrate. The matter, however, is complicated by 
the matter of water supply to the roots, and other consid- 
erations. 

3. Light affects plants complexly. Indirectly, of course, 
it is essential to green plants through its relation to food sup- 
ply. Directly, however, it has no great effect unless very 
strong, when it tends to check the growth of most plants, 
which can thrive best under some shade, as we have earUer 
noted (page 95). The records of auxographs show very 



Ch. IV, 7] 



GROWTH OF PLANTS 



159 



clearly that most plants, 
when temperature is the 
same, grow faster at night 
than in daytime ; and this 
fact is familiar to garden- 
ers. This greater growth 
at night, however, is chiefly 
a result of the fall in tran- 
spiration which accompanies 
darkness, but secondarily 
there seems also involved 
some release from a direct 
check imposed upon growth 
by very bright light. In 
such a case the light is sup- 
posed to act through the 
unfavorable chemical influ- 
ence of the blue rays upon 
the Hving protoplasm, — an 
action which, in forms Hke 
the Bacteria, where the pro- 
toplasm is unshielded by 
chlorophyll or other color- 
ing matters, makes strong 
light actually germicidal. 
But if plants are continu- 
ously exposed to too Uttle 
light, they tend to ''draw," 
as the gardeners say; that 
is, they become pale, in 
obvious partial starvation, 
and elongate greatly at ex- 
pense of other growth (Fig. 
111). This elongation has 
been commonly supposed 
to represent an adaptive 




Fig. 111. — Effect of different in- 
tensities of light upon the growth of 
Peas; X |. a, in darkness; b, in j 
light ; c, in full light. While for a time 
the growth in darkness is much the 
greater, the extra bulk is chiefly water, 
there is loss of dry weight, and the 
plant ultimately dies of starvation. 
(From Duggar.) 



160 



A TEXTBOOK OF BOTANY 



[Ch. IV, 7 



self-adjustment, tending to raise the green tissues into better 
light conditions, when the plants happen to start in dark 

places. 

4. Many minor condi- 
tions likewise affect growth, 
notably special gases in the 
atmosphere, peculiar condi- 
tions of soil and water, 
electrical currents, or baro- 
metrical pressure. Some of 
these conditions accelerate 
growth, while others retard 
it, though mostly in minor 
degree, and often in devi- 
ously indirect ways. 

All growth requires, in 
addition to warmth and hu- 
midity, a supply of water, 
food, and air. The water, 
absorbed by the roots, is 
needed for the swelHng of 
the young parts. The food, 
whether made in the green 
leaves or absorbed from 
some other source, is needed 
partly as building material 
for the enlargement of cell 
walls, and partly as a 
store of energy for effecting 
the plant work. The air, 
more properly the ox3^gen, 
is needed to release the 
energy in the food through 
the action of respiration, a 
fundamentally indispensa- 
ble process, considered in the following section. As by- 




FiG. 112. — The place of elonga- 
tion in growth of a typical stem of 
Lysimachia vulgaris, as shown by 
the spread of marks at first evenly 
spaced ; X ^. (Modified from Errera 
and Laurent.) 



Ch. IV, 7] 



GROWTH OF PLANTS 



161 



products of respiration, both carbon dioxide and heat are 
released in growth. 

All of the phenomena here described in connection with 
stems appear also in leaves, roots, and other parts, as shown 
by use of a suitably 
modified auxograph, or 
by the measuring mi- 
croscope. All parts are 
found to exhibit the 
grand period, and the 
fluctuations in relation 
to the weather; and 
the only appreciable 
differences in their 
modes of growth are 
associated with their 
differences in construc- 
tion. Stems grow by 
the progressive elonga- 




FiG. 113. — The place of expansion in 
growth of a typical leaf of English Ivy, as 
shown by the spread of the marks evenly 
made on the small leaf at the right ; X ^. 



tion of their internodes, which, forming in the bud, go 
through their grand periods in succession ; and thus is pro- 
duced the famihar appearance 
which has been compared with 
the opening out of the joints of a 
telescope (Fig. 112). The leaves, 
quite differently, expand pretty 
evenly throughout their structure 
(Fig. 113), going each through its 
grand period, as do flowers and 
fruits. The roots, on the other 
hand, grow almost solely near 
their tips (Fig. 114), their indi- 
FiG. 114. -The place of vidual cells passing very quickly 
elongation in growth of a typ- through the grand period, on 

icalroot of Corn, as shown by ^j^j^j^ aCCOUnt rOOts Seem as a 
the spread of evenly spaced 

marks ; X i- whole to grow evenly. This 




162 A TEXTBOOK OF BOTANY [Ch. IV, 8 

mode of growth for roots is fully explained by facts in their 
physiology and structure, as will later be noted. 

8. The Respiration of Plants 

This process has received frequent mention in the foregoing 
pages, and in ways which imply much importance. Since 
its phenomena are more clearly manifest in connection with 
growth than elsewhere, it may best be considered at this 
place. Respiration and photosynthesis are without question 
the two most important of all physiological processes. 

A demonstration of typical plant respiration is the fol- 
lowing. In the chamber of a respiroscope (a '^respiration 
demonstrator"), Hke that pictured in Figure 115, there is 
placed a handful of germinating seeds, and the instrument 
is stood in a warm, darkened place for twenty-four hours ; 
then water is poured down the thistle tube, and the air of 
the chamber thus forced out goes bubbling up through 
limewater placed in the cylinder. The limewater speedily 
turns milky, thus proving by this familiar test that carbon 
dioxide was present abundantly in the chamber. If the 
student should ask the natural question whether the carbon 
dioxide known to be present in air would not account for 
this result, the answer is given by the other or ''control" 
chamber, the air of which, sent through similar Hmewater, 
leaves it quite clear. In fact the quantity of carbon dioxide 
present in so small a volume of air is insufficient to show any 
effect by this method. It is therefore evident that carbon 
dioxide has been produced by the germinating seeds. If, 
further, a sample of the gas is withdrawn from the chamber 
containing the seeds and chemically analyzed, as can be 
done very easily, these additional facts appear. First, some 
of the oxygen originally present in the air has disappeared, 
and various evidence shows it has been absorbed by the 
seeds. In starchy seeds, like Oats, the oxygen absorbed and 
the carbon dioxide produced are approximatel}^ equal in 
volume, though often this respiratory ratio is different. 



Ch. IV, 8] 



RESPIRATION OF PLANTS 



163 



Second, not only carbon dioxide but also water is released 
by the germinating seeds. Thus in the growth of seeds, 




Fig. 115. — a respiroscope ; X i While the seeds are germinating, the 
chamber is kept closed by the smaller rubber stopper and clamp, though 
these are removed in applying the test. 

oxygen is absorbed and carbon dioxide and water are 
released. This, however, is precisely what occurs in the 
respiration of animals, including mankind. The process in 



164 A TEXTBOOK OF BOTANY [Ch. IV, 8 

the seeds is in fact respiration, identical in all essentials 
with that familiar in animals. 

If, now, other plant parts be placed in the respiroscope 
chamber, — growing bulbs, roots, buds, flowers, fruits, leaves, 
stems, — in fact any growing plant parts, and if the same 
tests and analyses as before be applied, then the general re- 
sult is always the same. It is true, there are cases in which 
oxygen seems to be absorbed without release of carbon 
dioxide, and cases in which carbon dioxide seems to be 
released without absorption of oxygen, with all intermediate 
gradations. These exceptions, however, from the point of 
view of respiration, are not real, for in the former case the 
carbon dioxide is known to be retained and used in forma- 
tion of special substances, and in the latter the oxygen is 
taken, not from the air, but from compounds composing the 
plant tissues themselves. And here is one further point of 
the first importance, fully proved by respiroscopes. Respira- 
tion occurs in general not only in tissues which are growing, 
but in all tissues which are alive. In brief, respiration is 
found to occur in all living and working parts of plants and 
animals alike. Its amount depends, as a rule, upon the 
activity of the parts, being directly proportional thereto. 

But what of green leaves, which were earlier shown to 
perform an opposite process in their photosynthesis, absorb- 
ing carbon dioxide and releasing oxygen? If, with green 
leaves in the chamber, the respiroscope be kept in the light, 
then indeed no carbon dioxide shows in the test ; but if the 
instrument be kept in the dark, then that gas is yielded 
abundantly. This does not mean that respiration stops in 
the light, but only that then it is surpassed by photosyn- 
thesis, which for the same area of leaf is, on the average, 
about twelve times more active than respiration. This fact 
explains why photosynthesis, even though in progress only 
a part of the time, can purify the air, despite the unceasing 
respiration of both plants and animals. 

Respiration is little affected by darkness or light. Ac- 



Ch. IV, 8] RESPIRATION OF PLANTS 165 

cordingly, since photosjoithesis is completely dependent 
on light, there must exist a certain light intensity, at which 
the photosynthesis and respiration of a leaf exactly balance 
one another. This must happen twice a day, morning and 
evening, and perhaps during very dark days. In such case 
it is probable that the two processes use reciprocally one 
another's products, and neither of the gases passes into or 
out of the leaf. It is because photosynthesis stops, while 
respiration continues, in the dark, that plants are unhealth- 
ful in sleeping rooms at night, though in truth the effect is 
quantitatively small, else one could not camp overnight in 
the woods. 

In respiration the oxygen comes from the air ; but the car- 
bon of the carbon dioxide, and the hydrogen of the water 
come from the food, in which they were incorporated, in the 
original grape sugar, by photosynthesis. Since respiration 
thus withdraws solid material from the body, it is always 
accompanied by loss of weight, and, unless compensated by 
addition of food, it ends in emaciation and starvation. 

Viewing the process now in the large, and centering atten- 
tion upon the end substances involved, we find that respi- 
ration may be expressed in an equation as follows., — 

C6H12O6 + 6O2 = 6 CO2 + 6 H2O. 

This equation, which may be termed the respiratory equation^ 
is the exact reciprocal of the photos3mthetic equation (of 
page 23). 

The exchange of gases here described is not, however, the 
important feature of respiration, but merely an incidental 
accompaniment thereof. The central fact, and explanation 
of the great physiological importance of the process is this, — 
that it releases the energy latent in potential form in the 
food, — which energy, set free at precisely the points of need, 
supplies the power which does the work of the plant in its 
growth and other processes. Respiration is in fact identical 
in nature with combustion, which, in a steam engine, pro- 



166 A TEXTBOOK OF BOTANY [Ch. IV, 8 

duces a union of oxygen with the carbon in coal, whereby, 
with formation of carbon dioxide, energy is released in 
the form of heat ; and this heat expands the steam which 
drives the engine and does the work. Both respiration and 
combustion are essentially energy-releasing processes, acting 
aUke through oxidation of carbon ; for it is one of the most 
fundamental of cosmic facts, that whenever and wherever 
carbon is allowed to unite chemically with oxygen, energy is 
set free, and can do work, whether in the cells of a plant, 
the muscles of a man, or the boiler of an engine. Respiration 
is in principle nothing but a slow regulated combustion 
within the bodies of plants and animals under protoplasmic 
machinery capable of turning its energy into work. 

What now is the original source of the energy thus con- 
tained in the food? Energy exists in nature in two forms, 
kinetic or active, hke heat, light, and electricity, and potential 
or latent, as in wound springs, raised weights, explosives, 
and unoxidized chemical substances in general. In fact, 
potential energy exists in all unsatisfied affinities, physical or 
chemical. When kinetic energy is used to wind springs, 
lift weights against gravitation, or separate substances from 
close chemical unions, it goes thereby into the potential 
form, and thus remains until the spring unwinds, the weight 
falls, or the chemical unions again take place, during which 
processes the potential energy again becomes kinetic, and 
can be used to do work, as clocks, water power mills, and 
artillery illustrate. The kinetic energy of the sunlight in 
photosjTithesis originally dissociated the carbon dioxide and 
water into their constituents, thereby passing into potential 
form. The oxygen was released into the air, but the carbon 
and hydrogen remained in the food, and thus were trans- 
ferred throughout the bodies of plants and animals, carrying 
the store of potential energy, which, on chemical access of 
oxygen in respiration, again becomes kinetic and does work. 
The process has an accurate parallel in the case of the storage 
battery, now familiar in the modern automobile. A charge 



Ch. IV, 8] RESPIRATION OF PLANTS 167 

of electricity, kinetic energy, is sent into the battery, and 
there forcibly dissociates certain stable chemical compounds 
into simpler substances; whilst these substances remain 
apart the energy is latent, ready to come forth once more 
in kinetic form (again an electrical current), when, by the 
closing of a circuit, the substances are allowed to re-combine 
into the old compounds. Food is nothing other than a stor- 
age battery, charged in green leaves by the sun, and discharged 
in the body by respiration. 

Thus it is plain why oxygen is necessary in respiration, 
and therefore why both plants and animals have need of 
''air." The parts of plants above ground have an ample 
supply admitted through stomata and lenticels along the air 
passages ; but the case is different with roots, for often the 
air is expelled from the soil by the presence of water. This 
is why wet soils must be drained, — not to remove water, but 
to introduce air. Where roots live continually in water, as 
in bogs and swamps, the air supply is usually obtained 
through large passages extending down through the stems 
from the leaves; and it is the presence of these ample air 
passages which gives the soft spongy texture to so many 
marsh plants. 

That respiration is indispensable to all plant processes in- 
volving growth and movement can be proven very strikingly 
by methods which deprive the parts of oxygen, while leaving 
them otherwise uninjured and ready for work. This is ac- 
complished by use of an instrument called the anoxyscope 
('^without-oxygen demonstrator") shown in our picture 
(Fig. 116). 

The energy released in respiration is mostly applied to 
various kinds of work involving motion, — the circulation 
of protoplasm, the enlargement of cell walls, and the like; 
and on the completion and cessation of motion it is con- 
verted into radiant heat. Thus all growing, and even all 
living, parts are somewhat warmer than their surroundings. 
A thermometer thrust into the opening flower of a Calla 



1 



168 



A TEXTBOOK OF BOTANY 



[Ch. IV, 8 



or a Jack-in-the-Pulpit will rise two or three degrees, and in 
some tropical plants of the same family, two or three times 
as much ; while in any living parts the release of some heat 




Fig. 116. — An Anoxyscope ; X i. The two tube-chambers are exactly 
alike except that from the one on the right all oxygen is removed by a 
chemical absorbent (pyrogallate of potash), which is replaced by pure 
water in the one on the left. In the case here shown, the soaked oats placed 
on wet moss in both chambers have grown well in one case but not in the 
other. The instrument permits many analogous experiments. 



Ch. IV, 8] RESPIRATION OF PLANTS 169 

may be proven by suitable methods, as explained with our 
picture (Fig. 117). It is this same respiration heat, intensi- 
fied and regulated, which yields the ''blood heat" of animals. 
It will now be worth while to summarize the contrast be- 
tween photosynthesis and respiration, as is possible in a 
table. 

Photosynthesis Respiration 

Occurs only in green plants Occurs equally in all plants 

and animals 
Takes place only in the chloro- Takes place in all living 

phyll grains protoplasm 

Proceeds only in light Proceeds in both light and 

darkness 
Constructs food Destroys food 

Increases weight Lessens weight 

Absorbs carbon dioxide Releases carbon dioxide 

Releases oxygen Absorbs oxygen 

Forms grape sugar from car- Reduces grape sugar to car- 
bon dioxide and water bon dioxide and water 
Stores energy Releases energy 

Thus respiration destroys the product of photosynthesis, 
but in the process develops the driving power for plant and 
animal life. 

There is a form or phase of respiration so important as 
to demand special treatment, and that is fermentation. 
If some Yeast (which is a unicellular microscopic plant, to 
be fully described in Part II of this book), in the form of 
ordinary compressed Yeast, be placed in a flask with water 
and sugar, and stood in a warm place, within a few minutes 
the mixture begins to ''work" or ferment, producing many 
fine bubbles which rise through the liquid and form a froth at 
the top. If, from this flask a bent outlet tube be led under 
limewater (Fig. 118), through which the escaping gas can rise, 
then the limewater will turn very milky, proving the gas 
to be carbon dioxide. After fermentation has ceased, the 



170 



A TEXTBOOK OF BOTANY 



[Ch. IV, 8 




Fig. 117. — A Caloriscope ; X i- The two 
Dewar bulbs have double walls with a vacuum 
between (as familiar in "Thermos" bottles), and 
hence prevent loss of heat. The respiring tissue to 
be tested is divided into two parts, and placed in 
the two flasks, but is immediately killed in one. 
Thus the delicate thermometers are made to record 
within a few hours the degree of heat released by 
the respiration of the living tissues. 



liquid in the 
flask is found to 
contain a con- 
siderable quan- 
tity of alcohol, 
together with 
small amounts of 
other substances 
known to be 
formed by the 
activity of the 
Yeast, which all 
this time is in 
vigorous growth 
and multiplica- 
tion. If certain 
mineral salts, es- 
sential to the 
metabolism of 
the Yeast, are 
present, the pro- 
cess continues 
until the sugar 
is exhausted, or 
else until the in- 
creasing quanti- 
ties of alcohol in- 
hibit the further 
growth of the 
Yeast. Thus, un- 
der experimental 
conditions, the 
Yeast plant in 
its growth fer- 
ments sugar to 
carbon dioxide 



Ch. IV, 8] 



RESPIRATION OF PLANTS 



171 



and alcohol, though it is ultimately overcome by the alcohol 
it produces. As to the significance of fermentation, all evi- 
dence unites to indicate that it represents primarily the 
Yeast's form of respiration, exaggerated usually by unnatural 
experimental conditions. Shut away from the air, the Yeast 
is unable to take oxygen from that source, and has to resort 
to the supply contained in the 
grape sugar, into which this plant 
has the power to convert cane 
sugar or starch. By action of an 
enzyme, the sugar is broken up, 
whereupon its carbon unites with 
oxygen from the same molecules 
into carbon dioxide, with the usual 
release of energy at the moment 
of union. The remainder of the 
molecule falls as a natural chemi- 
cal incident into alcohol, accord- 
ing to the following equation, — 

C6H12O6 =2C02+2C2H60. 



Thus 



m 



the formation of the ^^ 




Fig. 118. — Arrangement 
for demonstration of the re- 
lease of carbon dioxide in fer- 
mentation ; X 3. Explanation 
in text. 



carbon dioxide, a process essen- 
tial to the Yeast, the alcohol is 
incidentally and, so to speak, in- 
evitably formed. 

That fermentation is fundamentally the Yeast's respiration 
is attested by many lines of evidence, including the existence 
of intermediate steps between fermentation and ordinary 
respiration. Thus Peas, which ordinarily respire in the usual 
way, can also form carbon dioxide, with incidental production 
of alcohol, when derived of all free oxygen ; and this power 
is probably rather widespread, though in limited degree, 
among plant tissues which have imperfect access t6 air. 

The production of carbon dioxide and alcohol by fermen- 
tation renders the process of great importance to man. Thus 



170 



A TEXTBOOK OF BOTANY 



[Ch. IV, 8 




Fig. 117. — A Caloriscope ; X i- The two 
Dewar bulbs have double walls with a vacuum 
between (as familiar in "Thermos" bottles), and 
hence prevent loss of heat. The respiring tissue to 
be tested is divided into two parts, and placed in 
the two flasks, but is immediately killed in one. 
Thus the delicate thermometers are made to record 
within a few hours the degree of heat released by 
the respiration of the living tissues. 



liquid in the 
flask is found to 
contain a con- 
siderable quan- 
tity of alcohol, 
together with 
small amounts of 
other substances 
known to be 
formed by the 
activity of the 
Yeast, which all 
this time is in 
vigorous growth 
and multiplica- 
tion. If certain 
mineral salts, es- 
sential to the 
metabolism of 
the Yeast, are 
present, the pro- 
cess continues 
until the sugar 
is exhausted, or 
else until the in- 
creasing quanti- 
ties of alcohol in- 
hibit the further 
growth of the 
Yeast. Thus, un- 
der experimental 
conditions, the 
Yeast plant in 
its growth fer- 
ments sugar to 
carbon dioxide 



Ch. IV, 8] RESPIRATION OF PLANTS 173 

use the air, and even are fatally affected by its presence), they 
break up the molecules of organic substance, and therefrom 
obtain materials for their respiration and growth, simul- 
taneously forming various by-products. Usually they re- 
lease carbon dioxide in their respiration or fermentation, 
but sometimes they form other gases which happen to possess 
the offensive odors famiharly associated with decay. Some 
kinds of organisms star-t the decay, which other kinds con- 
tinue; and thus in successive steps the most comphcated 
organic substances are reduced gradually back to the car- 
bon dioxide, water, nitrogen, and mineral matters from 
which they were originally constructed. There is always 
at each stage some kind of organism ready to utilize any po- 
tential energy remaining in the organic substance, until 
finally all is exhausted. This is the ultimate fate of all or- 
ganic substance, which, formed photosynthetically in 
leaves from carbon dioxide and water and mineral salts, is 
converted back to those substances, either by the respiration 
of the higher plants and animals themselves, or else by the 
respiration of the micro-organisms of decay. Thus the 
carbon dioxide, water, nitrogen, and mineral matters with- 
drawn from the general circulation of nature and locked up 
for a time in the substance of plants and animals are all re- 
turned in time to their source. But on this withdrawal and 
return hinge the visible phenomena of life. 

The formation of incidental bj^-products by Molds and 
Bacteria in their respiration and growth has important prac- 
tical consequences to other organisms. Thus parasitic Fungi 
produce such by-products, whereby their host plants are poi- 
soned and damaged, often to complete destruction. The 
Bacteria associated with disease often produce violently poi- 
sonous products, collectively called toxins ; and it is these sub- 
stances, and not any direct injury done by the Bacteria them- 
selves, which cause death from bacterial diseases in man, other 
animals, and some plants. This fact underlies the methods 
of combating those diseases by use of antitoxins and the like. 



174 



A TEXTBOOK OF BOTANY 



[Ch. IV, 9 



9. The Geotropism of Stems 

When seeds are germinating, it is always found that no 
matter in what positions they happen to He, the sprouting 
roots grow over to point downwards, while the new stems as 
regularly grow upwards (Fig. 119). If, while these parts 
are still young, the germinating embryos are overturned, the 
roots and stems grow around into their former directions. 




Fig. 119. — Grains of Corn, fastened to vertical corks, with the root 
ends all pointing centerwards, and there germinated ; X I- The cork on 
the left was kept fixed in this position, but that on the right was kept re- 
volving, clockwise, on the clinostat (of Fig. 121). Since the roots grow much 
faster than the shoots, the latter were added to the drawings after the 
former. Note that on the fixed cork the roots point downward and the 
shoots upward, while on the revolving cork both roots and shoots continue 
the general directions in which they happen to start. (Drawn from photo- 
graphs.) 

Later, the side stems and roots as they develop assume 
positions which are horizontal, or nearly so ; and they like- 
wise, if overturned, grow again into those directions. If a 
young plant in a flower pot be laid on its side, its tip will 
be found next day to have grown around into the upright 
position ; and if the pot be completely inverted, then a day 
later the tip will again be found upright, now in exact re- 
verse of its original position in relation to the pot (Fig. 120). 
These adjustments occur in darkness as well as in hght, which 
shows that they are not phototropic (page 54), though like 



Ch. IV, 



GEOTROPISM OF STEMS 



175 



phototropism, they are growth movements, as experiment 
proves. If the plant be kept revolving upon a clinostat, how- 
ever (Fig. 121), then 
no such responses can 
occur, and the parts 
continue to grow in the 
directions they happen 
to have at the start. 
Everybody knows how 
stiffly upright are the 
Fir trees, and how 
remarkably horizontal 
their branches (Fig. 
122) ; and this is as 
true on the steepest 
hillsides as on level 
ground, showing that 
the upright position is 
not in any way deter- 
mined by the slope of 
the surface from which 
the trees grow. The 
facts here cited are 
typical, and represent 
a very widespread at- 
tribute of the higher 
plants, — that they 
grow in such manner 
as to swing their main 
roots straight down, 
their main stems straight up, and their side roots and stems 
at definite angles to the up-and-down direction. 

The up-and-down, and the horizontal, directions on the 
earth's surface are determined by a single factor, viz. gravi- 
tation, to which, accordingly, plants show remarkable ad- 
justments in their growth, — a property called geotropism. 




Fig. 120. — A Pelargonium, inverted 
and kept in the dark ; X ^. Note that the 
individual leaf blades have taken positions 
approximating towards horizontal. (Drawn 
from a photograph.) 



176 



A TEXTBOOK OF BOTANY 



[Ch. IV, 9 



It is clear that plants in growing geotropically are not forced 
into those positions by the gravitational attraction, for even 
if the young roots were pulled downward by gravitation, 
this force could obviously not push the young stem upward, 
or the side roots and branches out horizontally. In fact, 
geotropism bears the same relation to gravitation that 




Fig. 121. — A Clinostat ; X l- A powerful clockwork in the case on the 
right turns the central spindle continuously once in fifteen minutes. It 
can be used in any position. Thus plant parts can be kept revolving in any 
desired plane, whereby the action of a stimulus can be made equally all- 
sided instead of one-sided, — the most convenient method of neutralizing 
a one-sided stimulation. The instrument is much used in the study of 
irritability. 

phototropism does to Ught (page 54); just as the Hght 
neither pulls the stems towards it, nor pushes the roots 
away from it, nor forces the leaves across it, but acts simply 
as a guiding stimulus to the plant's own assumption of those 
positions, so gravitation neither pulls nor pushes the roots 
and stems into the positions they take, but acts simply as a 
guiding stimulus to the plant's own growth. This action 
of gravitation as a stimulus, and not directly as a force, ex- 
plains why parts grow as readily and perfectly away from 
it or across it as towards it. 



Ch. IV, 9] GEOTROPISM OF STEMS 177 

Seeking a connection between geotropism and the plant's 
physiology, we can find no direct relation, such as shows so 
clearly with phototropism in the relation between light and 
photosynthesis. It is true, under the influence of geotro- 
pism the roots are guided down into the soil, and the shoots 
up into the air, and therefore to the positions essential to 
their functions ; but, as one immediately recalls, those parts 
would take those positions without any geotropism, — for 
the shoot would be guided upward by phototropism and 
the roots downward by hydrotropism (an adjustment to 
moisture later considered). Geotropism, therefore, produces 
exactly the same results in shoots and roots as would photo- 
tropism and hydrotropism. But there is this difference, that 
the hght and moisture are variable, and often (especially the 
light) absent altogether at times when the plant is in growth 
and needing guidance, while gravitation is perfectly con- 
stant at all times. It seems wholly probable, therefore, that 
this invariability of action through all external conditions 
is a reason why plants employ gravitation as well as light 
and moisture in guiding the growth of their shoots and 
roots. Precisely the same principle is illustrated in our 
human affairs where we regulate our daily actions, our 
risings, our meal times, and our occupations, by the clock, 
instead of trusting to the height of the sun, often obscured, 
or our appetites, often spasmodic. In similar manner the 
sailor steers by his compass, even when sun or stars are 
visible. 

It must not, however, be inferred that growing towards 
gravitation or away from it are inseparable from the nature 
of a main root and main stem respectively. Thus, some 
main stems have the transverse or horizontal geotropism, 
as in ground vines. It is for this reason that Ground Pine, 
our creeping Ferns, and Solomon's Seal, keep their main 
stems down upon or just under the surface of the ground, 
instead of .sending them upwards. Moreover, the geotropism 
of parts can change during life, as familiar in cases where a 



178 A TEXTBOOK OF BOTANY [Ch. IV, 9 

side branch of Spruce or Fir turns upward, and takes the 
place of the main stem, on destruction of the main terminal 
bud. All evidence shows that the geotropism of any part is 
not in the least inherent in its morphological nature, but is 
correlated with its habit of life. 

Stems and roots are by no means the only geo tropic parts. 
Thus, in the experiment of the inverted potted plant, per- 
formed in the dark, not only does the stem grow upward, 
but most of the leaf blades become horizontal, through in- 
dependent readjustment upon their o\\tl petioles (Fig. 120). 
Something similar occurs naturally in the undergrowth of 
woods, where, in the evenly-diffused light, many kinds of 
leaf blades become strikingly horizontal. Flowers and 
fruits are often perfectly geotropic, as will later appear in 
connection with their functions. The lower forms of plants 
are likewise susceptible to geotropism; and the horizontal 
growth of the bracket Fungi upon trees, and the upright po- 
sition of the Mushrooms and Toadstools are thus guided. 

While geotropism thus guides the growth of plants in their 
various parts, its effects of course are intermingled with 
other irritable responses. Thus, when light acts from one 
side on a geotropically upright plant, the position which the 
plant actually takes is a resultant between its tendency to 
respond both ways ; and its roots and other parts are in- 
fluenced in similar manner. The fact is, the plant tends 
to guide its growth as a whole, and assume its general form, 
under the guidance of gravitation, thus establishing a gen- 
eral arrangement of parts, but this ground form is readily 
modifiable in details by other influences. 

It is interesting to note the way in which the plant appears 
to perceive, so to speak, the direction of action of gravita- 
tion. There is much evidence to show that in special cells 
of root and stem tips, the starch grains, which are relatively 
hard and heavy, press down by their weight on the sensitive 
protoplasm, and thus give the sense of direction, by which 
the growth processes are controlled. 



Ch. IV, 10] FOLIAGE-BEARING STEMS 179 

10. The Various Forms of Foliage-bearing Stems 

Stems which perform the same primary function, of sup- 
port to the foUage, can yet differ greatly in aspect and 
structure. These differences are connected with different 
habits. 

The simplest form of foliage-bearing stem is found where 
a vertical, cylindrical, unbranched trunk bears one set of 
leaves towards its summit, as do most Palms (Fig. 30). 
Such stems, which may be termed columna r, characterize 
crowded vegetation, as in the jungles of the tropics and 
the meadows of temperate regions. They often attain 
great heights in proportion to their diameters, in which 
case they exhibit very perfectly the hollow-column principle 
of construction (page 118). Thus the Bamboo, practically 
columnar though it bears small branches, presents an actual 
hollow column of such great strength as to give it high 
value for building and other uses in the tropics. 

The second form of stem is found where the leaves are 
spread along slender horizontal branches radiating from a 
columnar-conical trunk. In such cases the younger and 
shorter branches are above and the older and longer below, 
thus producing the very perfect cone-shape displayed in our 
Fir trees when free to grow without interference (Fig. 122). 
This form of stem, called botanically excurrent, is associ- 
ated particularly with the evergreen habit, in which the leaves 
persist and do some photosynthetic work in the winter. 
Evergreen trees are especially characteristic of high latitudes 
and altitudes; and it has been argued that the conical 
form, with the upper branches progressively covering the 
lower except near the tips, which readily yield to a weight, 
renders such trees largely immune against damage by the 
snow and ice of the chmates in which those trees most 
abound. When such trees are well spaced apart, the cone 
form is often strikingly perfect from ground to summit ; but 
when crowded together in forests, their lower branches are 



180 



A TEXTBOOK OF BOTANY [Ch. IV, 10 



overshaded, die, and fall awaj^, after which the old bases 
become buried as knots by the ever-increasing layers of the 
wood. Thus in time the cone of foliage comes to stand at 



^>. .. , 




Fig. 122. — Abies venusta, a Fir, showing the typical conical form. 
(From Bailey.) 

the summit of a branchless columnar trunk which yields 
our most valuable timber. Such stems do not show the 
hollow-column principle of construction, except incidentally 
when the heart wood is removed by decay, for the mode of 
growth is such as to build a stout solid column. 



Ch. IV, 10] FOLIAGE-BEARING STEMS 



181 



The third and most highly developed form of stem is found 
where a swelling dome of foliage is held outspread by a 
system of stems radiating, tapering, and branching from a 
relatively short central trunk, as well developed in our 
common deciduous forest trees, — Oak, Maple, Elm, and 
others (Fig. 123). Since the trunk thus melts away, as it 
were, to the twigs, the form 
is called deliquescent. It 
permits a great lateral 
spread as well as high eleva- 
tion of the foliage, and is 
the most effective of the 
forms in the exposure of 
great numbers of leaves to 
light; and correspondingly 
it is the dominant forest 
type the world over in both 
temperate and tropical re- 
gions. The damage which 
v/ould be entailed in win- 
ter on such widespreading 
forms by accumulation of 
snow and ice on the leaves 
is obviated by the shedding 
thereof in the autumn. Here, also, a remarkable symmetry 
of form is developed where space is sufficient, as Maples in 
old pastures. Elms in meadows, or lawn trees attest (Fig. 
124). In such cases the foliage has almost the form of a 
sphere, or an elhpsoid, or a rounded cone. A crowding in 
forests, however, produces the same effect as in the excurrent 
type, excepting that the branches here tend to reach up to 
a low dome or even flat top, as can be seen very clearly 
when a deciduous forest is viewed from some mountain. 
The effect is particularly striking where the dark spires of 
evergreens break through the rolling plain of the deciduous 
foliage. 







Fig. 123. — Sugar Maple, in a 
pasture, showing the deliquescent 
form. (From Bailey.) 



182 



A TEXTBOOK OF BOTANY [Ch. IV, 10 



V» if 












Vertical stems are radially symmetrical, or alike all around, 
both externally and internally. When horizontal or oblique, 
however, as with branches, they are often excentric, having a 
greater thickness of wood on the lower side, in obvious ad- 
justment to the support of the branch against the leverage 
imposed by its increasing length and weight. In such cases, 

also, there is frequently a thick- 
ening in the lower angle be- 
tween branch and stem, on 
the principle of a bracket, 
which is present in even 
greater development at a place 
of much greater strain, viz. 
at the angle where a verti- 
cal trunk joins spreading 
roots. Here the bracket often 
amounts to a buttress, as well 
shown in old Elms, and even 
more strikingly in some trees 
which grow in the tropics. 
The principle of resistance to 
strain explains also the form 
of the main branches in the 
deliquescent type, for com- 
monly they rise almost vertically from the trunk, and turn 
gradually outward, becoming vertical again at the foliage- 
bearing tip. 

Stems have not only to support the great mass of the foli- 
age and also their own considerable weight, but must like- 
wise resist lateral pressure from winds, which exert great 
power against the foliage and therefore strong leverage on 
; the stems. Corresponding thereto is the tough-elastic 
texture of the stems, whereby they are enabled to yield to 
winds in a manner to shed off their force, as one can see in 
. any great trees in a storm. Where strong winds prevail in 
one direction during the season of growth, a tree may be held 







Fig. 124. — The symmetry of a 
lawn tree, the weeping Birch. 
(From Bailey.) 



Ch. IV, 10] FOLIAGE-BEARING STEMS 



183 



so much of the time in the leeward position that it acquires 
a permanent set that way (Fig. 125), though the result is 
complicated by the greater transpiration, and consequent 
less growth, on the windward side. The leverage of the 
winds is felt most at the base of the trunk, which explains 
the need for the buttresses above mentioned. There is 
evidence to show that these buttresses, like the brackets 
and excentric growth of the branches, develop in irritable 
self -adjustment to the stim- 
ulus of the strains there felt, 
in precisely the same way 
that leaves and stems turn 
phototropically to light, or 
stems hold themselves up- 
right in adjustment to grav- 
itation. 

Between stems and 
branches no structural dif- 
ferences exist, the word 
''branch," as we use it, 
being merely an abbrevia- 
tion for '' branches of the 
stem." For the most part 

all of the branches of a given plant are structurally alike, but 
sometimes they are not. Thus in fruit trees, some branches 
make extremely little growth in length each year, while their 
buds form flowers and fruits with the least possible stem; 
and such branches are the famiUar fruit-spurs. Again, 
some of the branches on a plant may be limited in growth 
and assume flat forms, as in cladophylla elsewhere described 
(page 195), the remaining branches having the ordinary 
form. An even more familiar case of special branches is 
found in flowers, which are morphologically modified branches 
including sexual parts. In a few cases, trees form a certain 
absciss-layer across the bases of some of their young branches, 
producing the result of a natural pruning. 




Fig. 125. — A yellow Birch, ex- 
posed to winds from one direction 
during the growth season. (Drawn 
from a photograph.) 



184 



A TEXTBOOK OF BOTANY [Ch. IV, 10 



While the upright self-supporting condition is typical in 
foUage-supporting stems, modifications thereof occur in 
connection with special habits. Most prominent are climb- 
ers, which make use of trees, rocks, walls, and other supports 
to lift their foUage to the fight. Being thus supported, they 
need no great thickness and remain slender, devoting their 




Fig. 126. — A typical epiphytic Orchid, showing aerial roots, and the 
pseudobulbs, or storage stems, from which spring true leaves. (Reduced 
from Kerner.) 

material to increase in length. Some simply clamher over 
other plants, as in case of the Rattan Palm already men- 
tioned (page 113) or the many great lianas of the tropics, or 
the Clematis of our woods. Such plants possess hooks (Rat- 
tan), twining petioles (Clematis, Fig. 51), or other arrange- 
ments preventive of slipping from the supporting vegetation. 
Others, forming our principal vines, cling to a support, either 
by tendrils, as in Grape and Passion Vine (Fig. 136) , or by 



Ch. IV, 10] FOLIAGE-BEARING STEMS 



185 



adherent disks, as with Virginia Creeper, or by disks on the 
ends of aerial rootlets as in the Ivies which grow upon 
buildings (Fig. 180). Others are twiners, and wind their 
very slender stems around the support, as do Morning Glory 
and -Dutchman' s-pipe. Some special forms of irritability are 
concerned in the climbing moverrients. Thus, vines which 
climb against walls have the stems negatively phototropic, 
and thus are kept against the surface to which their 
roots adhere. 
All climbing 
stems remain 
slender, form- 
ing new wood 
but slowly, and 
possess, as a 
rule, very large 
ducts. 

From the 
climbing to an 
epiphytic habit 
there is every 
gradation in 
tropical vege- 
tation. Epi- 
phytes are 

plants which have no connection of their own with the ground, 
but live supported towards the Kght upon others, without 
being parasitic. Very few occur in the flora of temperate 
regions, aside from a few stray Mosses, Lichens, and other 
low forms, but most tropical Orchids, some Ferns, and 
many members of the Pineapple Family, including the 
''Long Moss" of the South, are typical epiphytes ; and they 
often cover the branches of tropical trees in great variety and 
profusion (Fig. 126). Their mode of life is pecuhar, and many 
striking adaptations thereto have been described by those 
who have studied them in the tropics. Their attachment to 




Fig. 127. — JSchmea miniata var. discolor, typical of 
the funnel-form epiphytes. (From Bailey.) 



186 



A TEXTBOOK OF BOTANY [Ch. IV, 10 



the supporting j)lant is precarious, and they remain compact 
with very short stems often concealed completely by crowded 
leaves. Their water supply comes from the rain which wets 
the bark on which their roots grow ; but a few possess methods 
of collecting the rain in funnel-shaped cups formed by their 

leaves (Fig. 127). All 
epiphytes, indeed, show 
marked water-conserving 
features, including thick- 
ened epidermis, sunken 
stomata, storage tissues, 
and other features associ- 
ated with plants which 
must stand frequent dry- 
ness (page 69). Their 
supply of mineral matters 
is such only as they can 
derive from the decaying 
vegetation amongst which 
they live, and much of it 
comes from the bark into 
which they send their 
roots. Some kinds, how- 
ever, collect among their 
leaves the bark, twigs, 
flowers, etc., which fall 
from above, while others 
possess leaves so adjusted 
to the supporting trunks 
as to form half cups in which bark and other materials 
streaming down with the rain are caught and held, later 
decaying to a humus from which both water and mineral 
matters are readily absorbed (Fig. 128). And many other 
interesting features, some structural and some self-adjustive, 
are known to accompany the epiphytic habit. From the 
penetration of dead bark for rain water to a penetration of 




Fig. 128. — An epiphytic Fern, Platy- 
ceriuyn grande, possessing two kind of 
fronds, — ordinary (drooping) and humus- 
collecting (upright) ; X j. (From Goebel.) 



Ch. IV, 10] FOLIAGE-BEARING STEMS 



187 



living stems for their soil water, the step would seem easy for 
roots ; and thus has probably originated the half -parasitic 
habit represented in the Mistletoe. Thence it is only a 
short step further to a connection with the food supply of 
the host plant, and a completely parasitic habit. It is prob- 
able that the parasitism of the flowering plants has mostly 
originated in this way. 

Like climbing stems in many respects are creeping or 
trailing stems, such as those of Partridge Berry and Ground 




Fig. 129. — The rhizome, or rootstock, with ascending shoots, of a 
Sedge ; X j. (From Le Maout and Decaisne.) 



Pine. Since the ground supports them, they remain slen- 
der, and simple in structure. This habit merges over imper- 
ceptibly into that where the stems run, not on the surface 
but just beneath it, as in some Ferns and the Grasses ; and 
remarkable self-adjustive adaptations have been described 
whereby the stems are kept at a constant depth. This habit 
is best developed in the Grasses and Sedges, where the slender 
underground stems branch and interlock so profusely as to 
form the familiar turf, from which rise short vertical stems 
bearing the foliage (Fig. 129). When thus underground, the 
stems lose their green color and acquire the aspect of roots, 



188 



A TEXTBOOK OF BOTANY 



[Ch. IV,10 



whence their botanical name of rootstocks; but they are 
always distinguished by the presence of nodes and rudi- 




FiG. 130. — Stolon of Black Raspberry. (From Bailey.) 



mentary scale-like leaves. Such rootstocks often accumu- 
late food, thus tending towards new organs, which we may 
best consider in the following section. 

There also occur a kind of traveling stems. The very 
slender woody stems of the Brambles bend over and touch 




Fig. 131. — Sempervivum. soboliferum, showing typical offsets. 
(From Kerner.) 

the ground at their tips, where they take root ; and thus the 
plants form dense and ever advancing thickets (Fig. 130). 



Ch. IV, 10] 



FOLIAGE-BEARING STEMS 



189 



Some plants develop both upright and reclined stems, the 
latter, called stolons, lying close to the ground, as in 
Hobble-bushes, descriptively named. Short leafy stolons, 
called OFFSETS, are formed by some plants of compact 
growth like the Sempervivums, which thereby spread out- 
ward in a continuous 
growing mat (Fig. 
131). Very long and 
slender stolons, evi- 
dently adapted to 
spreading the plant, 
are called eunners, 
as familiar in the 
Strawberry. 

The flowering 
plants are typically 
land dwellers, but in 
course of their evolu- 
tion some kinds have 
returned to a life in 
the water, — e.g. 
Water-lilies and a 
great many of the 
Waterweeds. The 
stems of such plants 
are buoyed up by 
the water, which 
thus supplies the 
support for the foli- 
age, in correspond- 
ence wherewith the stems are weak and soft, serving rather 
as cords to retain the leaves than columns to lift them. 

Some flowering plants hve also in deserts, into which they 
have been forced in the course of evolution. The scarcity 
of water entails on such plants great reduction of surface, 
leading in the most typical cases, like the Cactus, to aban- 




FiG. 132. — Fucus vesiculosus, the common 
brown Rockweed ; X y. (From Figurier.) 



190 



A TEXTBOOK OF BOTANY [Ch. IV, 10 



donment of the leaves and the assumption of photosynthesis 
by the compact, rotund, water-storing, ribbed stems, which 
possess many structural features connected with restriction 
of transpiration (Fig. 141). The difference in aspect and 
structure between forest plants, desert plants, and water 
plants shows how profoundly plant form is affected by water 
supply. In accordance, indeed, with this relation to water, 
most plants fall under three well-recognized groups, the 
desert plants being called xerophytes, the water plants 




Fig. 133. — The Giant Kelp, Mac- 
rocystis pyrifera, which grows up- 
wards of 200 feet long. (From 
Le Maout and Decaisne.) 



HYDROPHYTES, and the intermedi- 
ate or ordinary plants mesophytes. 
The mesophytic is of course the 
best condition for plant life, and reaches its highest perfec- 
tion in the rank growths of the tropical forests and jungles, 
though it is nearly as well attained in the deciduous forests 
of temperate regions. 

The primitive water plants, the Algae, in their highest 
development are distinguished by a thallus, famihar in 
the fronds of brown Rockweeds (Fig. 132) and the red Sea- 
mosses. The thallus is neither leaf nor stem, but rather a 
more primitive structure from which leaf and stem have not 
yet differentiated. Some of the greater Algae, as for ex- 
ample the giant Kelp of the Pacific (Fig. 133), have de- 
veloped a distinct leaf and stem structure, though it by no 
means represents the evolutionary ancestor of the shoot of 
the higher plants. 

The term shoot is used in connection with the flowering 
plants to designate stem and leaves collectively. 



Ch. IV, 11] SPECIAL FUNCTIONS OF STEMS 



191 




Fig. 134. — Solomon's Seal, Polygonatiim 
multiflorum ; X f. Each "seal" marks a fallen 
shoot, and a year's growth of the rootstock. 
(From Strasburger.) 



11. The Forms and Functions of Stems not Connected 
WITH Support of Foliage 

As with other 
plant parts, stems 
are not Hmited to 
the one primary 
function in adapta- 
tion to which they 
seem clearly to have 
been evolved, but 
perform also others, 
which sometimes re- 
place the original 
function. Thus are 

produced new organs, with distinctive aspect and structure. 
The most frequent additional function of stems is storage 
of food or water. All woody stems store 
food over winter, but since ample room 
therefor exists in the ordinary tissues, — 
in pith, bark, medullary rays, and parts of 
the fibro-vascular bundles, — such stems 
exhibit no external evidence of the storage 
function. Some stems, however, do show 
marked swelhngs resulting from storage of 
food and water, as especially clear in the 
pseudobulbs of epiphytic Orchids (Fig. 
126). Storage of food is commonest in 
underground stems or rootstocks, which 
thereby are given a swollen aspect, as for 
example in Solomon's Seal (Fig. 134), 
where a new piece of food-filled stem, 
producing a new shoot, is made each year. 
Similar arrangements are found in Iris, 
Trillium, and others, and reaches an ex- 
treme in the corm of Crocus (Fig. 135), 




Fig. 135.— Atyp- 
ical corm, composed 
mostly of stem, of 
Crocus. (From 
Figurier.) 



102 



A TEXTBOOK OF BOTANY [Ch. IV, 11 



where the nearly globular storage stem is commonly mis- 
taken for, and called, a bulb (page 73). All of these 
stems produce roots, and also give rise to the foliage; 
but cases occur in which food-storage completely displaces 



the foUage-supporting function, 
and also the production of roots. 
Then we have a new organ, ex- 
emphfied in the common potato, 
the stem nature of which is 
attested by the eyes, which are 
axillary buds subtended by small 
scale leaves. Such an organ, 
rotund with accumulated food, 
and composed mostly of thin- 
walled rounded storage cells of 
the greatly developed pith and 
cortex, is called a tuber, of which 
many forms occur among plants. 
Another important special func- 
tion of stems is represented in 
tendrils, which have the same 
elongated slender forms, move- 
ments through the air, thigmo- 
tropic twining about a support, 
and spiral shortening, already 
described in leaf tendrils (page 
77). Passion Vine, Wild Cu- 
cumber, and Grape Vines have 
stem tendrils (Fig. 136), which are more abundant and 
perfect in form than leaf tendrils, perhaps because support 
is a more natural function of stems than of leaves. 

Stems also become transformed into spines, which are 
sometimes very large, as in Honey Locust (Fig. 137). The 



Fig. 136. — Tendrils, from 
axillary buds, in a Mexican 
Passiflora. 

Compare also Fig. 52. The 
tendrils of Grape Vine and all 
of the Gourd family (Squash, 
Wild Cucumber) , represent the 
main stem, the further growth 
taking place from the axillary 
bud. (After Gray.) 



Ch. IV, 11] SPECIAL FUNCTIONS OF STEMS 



193 



single spine of the Cactus-like Euphorbias is a stem, really 

the persistent and hardened flower-bearing branch. As in 

case of leaves, however, 

the significance of these 

spines is uncertain (page 

79). 

Support of the flowers, 
which mostly stand out 
in the light, is another of 
the special functions of 
stems. Flower stalks 
are usually slender-cylin- 
drical, nodeless, and leaf- 
less, though sometimes 
they bear bracts (page 
73). An elongated stem 
ending in a single flower 
or small cluster, espe- 
cially if starting directly 
from the ground, as with Adder's-tongue or Violets, is called 
a SCAPE ; a flower stalk from the axil of a leaf is called a 

PEDUNCLE, and in clusters 
each separate stalk is a 
PEDICEL. A typical flower 
stalk consists really of one 
internode, bearing at its 
top several nodes merged 
together in one enlarged 
RECEPTACLE wMch Sup- 
ports the floral parts (page 
271). 

The most striking of the 
new functions assumed by 
stems is found in the re- 
placement of leaves as 
foUage. In the simplest 




Fig. 137. — Spine, a branch 
developed from an axillary bud, 
in Honey Locust ; X r* 




Fig. 138. — Rubus squarrosus, a 
shrub in which the foliage function is 
assumed by the stems and petioles ; 
much reduced. (From Wiesner.) 



A 



194 



A TEXTBOOK OF BOTANY [Ch. IV, 11 



case the stem acquires more chlorophyll, shown by a deeper 
green color, thus supplementing better the work . of the 

leaves ; but in others the leaves 
are reduced in size almost to dis- 
appearance, leaving the foliage 
function wholly to the slender- 
cylindrical stems and petioles. 
In others the stems become 
flattened, thin, and green like 
the leaves, as in the familiar 
greenhouse plant Muehlenbeckia 
(Fig. 139), the stem nature of 
which, despite its deep green 
color, is proven by the prominent 




Fig. 139. — Muehlenbeckia platy- 
clada; X |. (From Goebel.) 

nodes and the persistent 
small leaves. Still more 
striking are the cases in 
which flattened stems, in 
this case branches, be- 




FiG. 140. — Leaf-like cladophylla 
(branches) of Butcher' s-B room, Ruscus 
Hypoglossum, in the axils of bracts, and 
bearing leaves and flowers ; X |. (After 
Kerner.) 



come limited in growth, 

and assume characteristic 

leaf shapes, to such a degree that their stem nature would 

hardly be suspected at all, were it not that they grow from 



Ch. IV, 11] SPECIAL FUNCTIONS OF STEMS 



195 



the axils of small scales which are morphologically leaves, 
as exemplified in the famihar " Smilax" of the florists. The 
Butcher's-Broom of Europe is similar in general, but has 
this further interesting feature, that on the face of the 
CLADOPHYLL (as such leaf -like branches are called), occurs 
a small though genuine leaf, bearing in its axil a flower 
cluster (Fig. 140). The apparent leaves of the common 
'^ Asparagus Fern" likewise 
are branches, of which several 
occur in the axil of each 
scale-like leaf. In clado- 
phylla the stems have be- 
come foliage without other 
function. 





Fig. 141. — Echinocactus, a 
typical globular ribbed Cactus, 
(Originally after Engelmann.) 



Fig. 142. — Rhipsalis Houl- 
letii; X 2- The seeming leaves 
are flattened stems, morpho- 
logically equivalent to a form 
like the Echinocactus of Fig. 
141, with the ribs reduced to 
2 and flattened. (From Riimp- 
ler, Die Sukkulenten.) 



The functions of foliage and storage are combined in the 
succulent stems of Cactus and other plants of dry places. 
Such stems, which store principally water absorbed during 
the rainy season, become swollen to cylindrical, or even 
almost globular forms, while the entire leafless surface bears 
ample chlorenchyma, with stomata through the thick epi- 
dermis (Fig. 141). Many of these plants possess vertical 



196 A TEXTBOOK OF BOTANY [Ch. IV, 12 

ribs, which have the effect of increasing the spread of green 
surface without a proportional increase of transpiration, 
which, of course, is the ever-present danger to plants of dry 
places (page 69). These ribs vary much in number, from 
many to few, and even in some cases to two, when the struc- 
ture approximates closely in appearance and function to a 
single leaf (Fig. 142). Thus is presented still another exam- 
ple of the attainment of the same functional end by a dif- 
ferent morphological route. 

The explanation of such remarkable morphological-physi- 
ological overturnings as are presented by the cladophylla 
is probably to be found, as with similar anomahes in leaves, 
in a devious course of evolution through conditions and 
habits very different from those now distinctive of these 
plants. 

12. The Monstrosities of Stems and Leaves 

It often happens that individual parts of plants grow so 
differently from their usual method as to attract attention 
and be designated ^'freaks." Scientifically such cases are 
called abnormalities, or if extreme, monstrosities. Aside 
from their interest as curious things needing explanation, 
they are scientifically important for the light they throw 
upon the methods of plant development. 

First, it must be noted that not all peculiar growths are 
properly monstrosities, for many result from purely mechani- 
cal causes. Thus, when a stem is encircled by a rigid ring 
(e.g. supporting iron band or wire attachment of a label), 
it becomes thereby constricted in its further growth, and 
swells greatly above the obstruction, because of the ac- 
cumulation of food stopped in its downward passage through 
the bark (Fig. 107). Precisely this cause produces great 
spiral ridges on trunks gripped by twining vines. Again, 
different parts of the same plant often become grown or 
grafted together, because crushed or rubbed against one an- 
other when young. In this way twin fruits are sometimes 



Ch. IV, 12] MONSTROSITIES OF STEMS 



197 



I 



produced, though others are true monstrosities resulting 
from partial fission of one. Oranges sometimes exhibit a 
segment very different in color and texture of skin from 
the rest ; but these are a special incident of grafting, as else- 
where explained (page 211). Strawberries which remain 
hard, shrunken, and green on one side are merely individuals 
which did not receive enough fertiliz- 
ing pollen (page 279). And other 
peculiarities of like sort, more or less 
obvious in origin, occur in various 
plant parts. 

Of true stem monstrosities perhaps 
the most common are fasciations. 
These are cases in which the usually 
cylindrical stem with its single ter- 
minal bud becomes a flattened stem 
with several imperfectly separated 
terminal buds, as occurs at times in 
Asparagus (Fig. 143), Hyacinths, and 
other herbs, and in Forsythia and 
Barberry among shrubs. A striking 
example, seemingly in a fruit, but 
really in a stem, occurs in the Pine- 
apple figured herewith (Fig. 144). 
Fasciations are much more common ^^ normally cylindrical; 

IX- - J XI, • Mj 1 X J X 2- (Drawn from a pho- 

m cultivated than m wild plants, and tograph.) 
sometimes can be propagated ; as, for 

instance in the Crested {i.e. a fasciated) Cactus (Fig. 145), 
while a crested form of Celosia gives us the Cockscomb of 
our gardens, and a related condition in leaves produces the 
feathered fronds of the Pearson Fern, — a new variety of the 
plain Boston Fern. Fasciations are evidently caused by a 
partial fission of one meristematic growth center into several. 
In some cases the result follows an injury by insects, but in 
such cases it cannot be propagated ; in others it seems clearly 
due to internal causes of still unknown nature, affecting the 




Fig, 143. — Fasciated 
shoot of Asparagus, which 



198 



A TEXTBOOK OF BOTANY [Ch. IV, 12 



meristematic tissues or the reproductive cells, and these are 
the kinds which it is possible to propagate, and thus preserve 




in our gardens. 




Fig. 145. — Greatly fasci- 
ated, or crested, Echinocactus 
(From Rtimpler.) 



Fig. 144.— 
A Pineapple, 
fasciated to 
an unusual 
degree. It is 
flattened in 
the plane that 
is visible; xf. 
The Pineap- 
ple is mostly 
stem covered 
with coales- 
cent small 
ovaries and 
bracts. 

(Drawn from 
a photo- 
graph.) 



The first step towards a fasciation would 
be a bifurcation, sometimes seen 
in the fronds of Ferns, and in some 
double fruits, i.e. in Orange (Fig. 
146). 

Closely related to f asciations are 
cases of unregulated hud develop- 
ment, most familiar in the Bird's- 
eye Maple. The eyes are knots, 
that is, buried branches, developed 
from a mass of adventitious buds 
which start on the side of a trunk 
of a Maple, presumably as a result 
of some injury (page 137), and in 
their growth about keep pace with 
the expansion of the trunk. An- 
other prominent case is found in 
''Witches' brooms" (Fig. 147), 
those dense masses of slender twigs 
found on the upper branches of 



Ch. IV, 12] MONSTROSITIES OF STEMS 



199 



Spruces and some other trees. Here, instead of the usual 
development of a few buds with inhibition of others, many 
or all of the buds on the 
branches affected develop 
equally, and more or less 
independently of the 
others. It is known 
that this condition is 
produced by the pres- 
ence of a parasite, the 
obvious effect of which 
is to paralyze the mech- 
anism of growth correlation by which the buds are ordinarily 
controlled. 

Closely analogous to these cases in buds is the unregulated 
growth of tissues. Thus, the large burls or gnarls which ap- 




FiG, 146. — A twin-fruit, of Mandarin 
Orange ; X \. (Drawn from a photo- 
graph.) 




Fig. 147. — A typical Witches' Broom, caused by an ^cidium, a Fungus, on 
a branch of Fir. (From Kerner.) 



200 



A TEXTBOOK OF BOTANY [Ch. IV, 12 



pear on old Elms, especially near the bases of the lower 
great branches, are composed of complexly contorted ■ and 
twisted masses of wood, often beautifully grained when 
sectioned and polished. They are formed by areas of cam- 
bium, which, instead of keeping their places and parts in the 
regular fibro-vascular cylinder, proceed to grow profusely, 
and thus are thrown out into irregular folds. A less extreme 
case is found in Curly Birch, and in some other irregularly 
grained hardwoods highly valued in fancy carpentry. In 

some cases such growths 
are apparently started by 
injurious strains, which 
would explain their fre- 
quency at the bases of 
great branches ; and very 
likely they represent areas 
in which the growth-con- 
trol mechanism has been 
ruptured by the strain. 
It is interesting to note 
that a close analogy exists 
between these burls and 
the troublesome tumors 
which form in the human 
body, for the latter also 
are formless growths re- 
sulting from continued operation of the growth energy of 
the tissues after the control stimuli have been inhibited, 
usually as result of some strain or other accident. Other 
burls, however, with various kinds of knotty growths, are 
started by presence of parasites, which also inhibit the 
usual control, presumably by chemical action. Of this 
nature is the remarkable "wooden flower," sold to tourists 
in tropical America (Fig. 148). It is nothing but a stem in 
which a parasite has inhibited the growth control over a 
limited area, leaving that part free to grow as it happens. 




Fig. 148.- 
Wooden Rose 



- A Wooden Flower, or 
on a leguminous plant ; 



X J. The parasite which induced it was 
a flowering plant, Phoradendron. (From 
Engler and Prantl, Pfianzenfamilien.) 



Ch. IV, 12] MONSTROSITIES OF STEMS 



201 



Related to these pecuHarities of tissue development are the 
TORSIONS, or close twistings sometimes found in plant tissues, 
either stems or fruits. They are often prominent on trees 
standing in burnt woods, or on fence rails, where the layers 
of wood form closely wound spirals. 

Rather striking, and not uncommon, are proliferations, 
well illustrated in the cases where a leafy shoot projects 
from the tip of the fruit in Pear or Straw- 
berry (Fig. 149). In Roses the stem occa- 
sionally grows up through the center of a 
flower and produces another, thus making 
a "two storied" flower (Fig. 150), while 
two-storied fruits, of similar origin, occur 
occasionally in Apples. An incomplete 
case is represented in the Navel Orange, 
where the stem grows up between the seg- 
ments of the fruit, and bears a smaller 
orange, not, it is true, on the top, but 
within the top of the main one. This case 
is also of interest as showing that such 
monstrosities can be propagated, for all 
Navel Oranges are reproduced by grafting. 
Stems, and therefore the stalks of flowers pj^^ ;^49 _ p^.^, 
and fruits, can potentially elongate indefi- liferous Pear. (From 
nitely, and some special inhibitory influence 
must ordinarily check their growth in flowers and fruits. It is 
apparently the occasional failure, presumably by some acci- 
dent, of this inhibitory stimulus, which results in prohferations. 

Among the commoner monstrosities are substitutions 
of one part or feature for another. Most people know that 
green Roses occur; and a variety is grown in Botanical 
Gardens on which the flowers are well-nigh as green as the 
leaves. Formerly such cases were considered '^reversions,'' 
the petals being supposed to have returned to the state of 
green leaves from which they were evolved. They seem 
rather, however, to result from a substitution of chlorophyll 




202 



A TEXTBOOK OF BOTANY 



[Ch. IV, 12 



for the usual color substance, of which the formation is 
inhibited by some accident. We sometimes find the oppo- 
site phenomenon, where the 
floral color is thrown into 
leaves, as happens with some 
Tulips, in which the upper- 
most leaf of the flower-stalk 
takes the color of the flower. 
Genuine reversions no doubt 
do occur; and perhaps we 
have a case in the occasional 
appearance of leaves upon the 
smooth sides of Apples and 
Cucumbers, this part of the 
fruit being morphologically 
stem. Sometimes Potatoes 
appear above ground in the 
axils of the leaves, evidently 
because food material destined 
for the underground tubers 
becomes diverted into axillary 
buds. 

There can be little doubt that with increasing knowledge 
we shall learn to control such substitutions, and various other 
stimuli which produce special growths upon plants. Thus the 
horticulture of the future will surely include some practice 
whereby palatable and nutritious growths, on the analogy of 
aerial tubers and galls, will be produced at will upon the 
leaves or stems of plants. 

Several forms of monstrosities are distinctive of leaves. 
Rather common is the formation of a cornucopia-like pitcher, 
instead of a flat blade, as happens in Pelargoniums, Cabbage, 
and others (Fig. 151). Here the bases of the leaf blade seem 
to unite or graft together over the petiole at an early stage, 
and remain united during the subsequent growth. The 
case has an interest in showing one way in which pitchers 




Fig. 
(From 
tology.) 



150. — Proliferous Rose. 
Masters, Vegetable Tera- 



Ch. IV, 12] MONSTROSITIES OF STEMS 



203 



may have originated in the Pitcher Plants (page 76). Also 
distinctive of leaves is a peculiar monstrosity called phyl- 
LOMANiA, propagated in a green-house variety of Begonia, 
where the stem or petioles produce a great number of very 
minute, but otherwise well-formed blades (Fig. 152). Here 
the form-factors which shape the blade, whatever they are, 
evidently have spread all over the plant. An extremely fine 
division of the leaf blade, closely following the veins, some- 
times occurs, and can be propa- 
gated : and such is the origin 
of the '^laciniate" or finely 
cut leaves of some cultivated 
trees and shrubs. 

Not properly monstrosities, 
though usually associated and 
intergradient therewith, are 
GALLS. Typical examples 
occur in the bright red round 
swellings on Oak leaves, which, 
when opened, are found to 
contain the larva of an insect 
(Fig. 153). A common form 
upon stems is the familiar 
globular swelling of the stem 
in Golden Rods. They are 
formed by the plant tissues 
after an insect has laid an egg 

therein, though we do not yet know the precise nature 
of the stimulation which controls their development. The 
growing insect feeds upon the leaf tissue, then makes its 
way out and escapes. The advantage of the arrangement 
to the insect is plain, but its meaning to the plant is still 
problematical. Hundreds or thousands of such galls are 
known, constant in form for the same kind of insect on the 
same kind of plant. Some are large, some small, some rough 
or hairy, some smooth, some on leaves and some on stems, and 




Fig. 151. 



— Abnormal leaf of a 
Potentilla. 



204 



A TEXTBOOK OF BOTANY [Ch. IV, 12 




Fig. 152. — Begonia phyllomaniaca, which produces many small leaves 
over leaf and stem. (From Bailey.) 

some involve both, as in case of the Willow Roses, — those 
rose-like masses of shortened leaves often seen on the ends of 
Willow stems. 




Fig. 153. — Typical galls, with the Insects, of Oak; slightly reduced. 

On the left a leafy "Oak-apple," and on the right the insect in cocoon 
and adult stages. In the center, an Oak gall, and on the right, lower, the 
same cut open, showing the larva of the insect. (From Thome, Text-hook 
of Botany.) 



Ch. IV, 13] ECONOMICS OF STEMS 205 

A very close relation exists between monstrosities, and 
those extreme variations called in horticulture spokts. 
In fact a sport, the foundation of some of our most valuable 
varieties of cultivated plants, as typified, for example, by the 
Navel Orange, is probably nothing other than a monstrosity 
which has originated from internal and not external causes, 
and which can be propagated. 

Monstrosities occur, of course, in the other plant parts, 
notably flowers and fruits, and along with our description 
thereof we shall consider still further their causes. 

13. The Economics, and Treatment in Cultivation, of 

Stems 

As with other plant parts, stems possess structures and 
contain substances suited to their functions and habits. 
These materials, however, happen to meet certain needs of 
man, who accordingly appropriates them for his purposes. 

The size, composition, and tough grain of the great 
trunks built by trees for support of their foliage fit them ad- 
mirably for innumerable domestic and manufacturing utili- 
ties. Nature has suppUed lumber and cabinet woods in 
great abundance and variety, but not so great as man's 
increasing needs; and he is driven perforce to conserve, 
augment, and improve the supply through scientific forestry. 

Likewise from stems he obtains material for paper, not now 
as in old times from consolidated strips of herbaceous pith 
(papyrus), but from cellulose fibers (rag or linen papers), 
and from the lignified elements of the xylem. These he sep- 
arates by grinding, or else by use of chemicals which dissolve 
the middle lamellae (page 147), and then felts them together 
to a pulp which is compressed between rollers to the familiar 
thin sheets. Also he uses tough bast fibers for threads, 
notably in case of Flax, which he weaves to cloth, giving 
linen, though cotton has a very different origin, as will 
later appear. Both bast fibers and sclerenchyma strands 



206 A TEXTBOOK OF BOTANY [Ch. IV, 13 

are utilized as hemp, or other cordage. Likewise the bark- 
cork has uses dependent on its waterproof quahties. 

From the stores laid down by plants in their stems man 
derives many foods, either directly through some vegetables 
or indirectly through fodder plants. Most of his sugar 
comes from the main stems of the Sugar Cane, and a Httle 
from Maple, and some starch from Sago Palm, while special 
storage stems, like potatoes, yield him specially rich harvest. 
And Ukewise from stems he draws drugs, dyestuffs, tanning 
substances, resins, rubber, and almost innumerable other 
materials, having in the plant distinctive meanings which 
involve properties happening to serve some human 
purpose. 

Man's command over the resources of Nature rests not 
alone upon his direct appropriation and use of materials 
which plants happen to offer, but also upon his power to 
multiply their quantity and improve their quality by culti- 
vation. That part of cultivation which consists in conform- 
ity to the plant's physiological pecuharities (page 94) is 
comparatively simple with stems, involving no special hor- 
ticultural or agricultural practice, doubtless because of the 
relatively simple and mechanical part taken by stems in 
the plant's economy. But the other phase of cultivation, 
viz. improvement, which always depends on the utilization 
of potentialities which the construction or composition of 
the plant happens to offer, has some important applications 
in stems, especially in connection with pruning and grafting. 

Pruning consists in the removal of some parts of a plant 
for the benefit of the remainder. Its very possibility de- 
pends on two leading facts. First, branches are practi- 
cally all repetitions of one another, and hence are not in- 
terdependent ; and accordingly any particular ones may be 
removed without damage to the rest. Second, any injuries 
made in living tissues of plants not only heal quickly, but 
the bark gradually overgrows and permanently covers large 
areas of dead tissues, as already described (page 122, Fig. 79). 



Ch. IV, 13] ECONOMICS OF STEMS 207 

If pruning is done in winter or early spring, the injuries heal 
largely before the first rush of the valuable sap. 

There are four principal uses of pruning. First, parts 
affected with disease which might spread to sound parts 
can be removed. Second, some desired shape can be given 
ornamental or fruit trees by removing growth in undesired 
directions. This practice merges over imperceptibly into 
the clipping of plants forcibly to desired shapes, as practiced 
with hedges or with evergreen plants in the topiary work 
of formal gardens. Third, more space and light can be 
insured to a few branches, in place of a mediocre exposure 
to many, thus promoting the development of fine individual 
flowers or fruits. Trees and shrubs not only form many 
more buds than ever develop, but develop many more 
branches than is good for them all. By a form of pruning, 
viz. disbudding, it is possible to develop the wonderful great 
exhibition types of Chrysanthemum. 

The fourth use of pruning is the most important of all, 
especially in orchards, — viz. to produce more formation 
of fruit and less of leaf and stem. The possibihty of gaining 
this end by pruning depends on the fact that in plants (as 
also in animals) a certain reciprocal balance exists between 
the reproductive and the vegetative parts, such that any 
check to either promotes the other, — and the fruit, of course, 
is a part of the plant's reproductive mechanism. In a state 
of nature, all woody plants form only enough reproductive 
parts for their needs, and, as a phase of their competition 
with one another for light and space, throw the remainder 
of their energy into growth of stem and leaf. The human 
fruit-grower, however, does not so much wish his trees to 
become big as to bear plenty of fruit; and by pruning 
away much stem and leaf, he can turn the plant's energy 
into more copious formation of fruit. Thus the cultivated 
Grape Vines, left to themselves, produce long leafy canes 
bearing few clusters of Grapes ; but when thoroughly pruned, 
they produce Httle cane but many fine clusters. Of course 



208 A TEXTBOOK OF BOTANY [Ch. IV, 13 

such pruning must be done with discretion, for in the last 
anal3^sis the production of fruit depends upon the work of 
leaves and stems ; but the aim of the pruner is that optimum 
balance at which only enough food is sent to stem and 
leaves to insure moderate growth for the next season, while 
all of the remainder goes into fruit. Naturally the best 
pruning requires judgment, skill, and technique, which 
are acquired only by a combination of natural aptitude with 
long and interested practice. 

There are other minor uses of pruning for special purposes, 
of which an example is the root-pruning said to underlie the 
production of the remarkable dwarf trees of the Japanese. 
By the consequent restriction of water and mineral matters, 
the entire development of the plant is restrained without 
other alteration of its characteristics. 

Even more important than pruning in the utilization 
of the natural potentialities of stems is grafting, or, as the 
entire art comprehensively is now often called, graftage. 
It consists essentially in this ; — a piece of stem, called a 
ciON, or SCION, of some valuable variety of plant is inserted 
into the stem of another, which is usually a less valuable but 
more hardy kind, called the stock, in such manner that 
the cambium tissues can unite. In these cases cion and 
stock grow together as one organism, which through life, no 
matter how large the plant becomes, retains below the 
union the hardy roots and other characters of the one, and 
above the union the special good qualities of the other. 
The possibility of grafting depends upon the capacity of 
the cambium of related plants thus to unite ; and its value 
depends upon the permanent retention of the characters of 
the cion substantially unaltered. 

In practice only closely-related kinds can be grafted to- 
gether, presumably because of chemical incompatibility in 
the protoplasm of more distant relatives. Further, only 
exogenous kinds will unite, because the joining of the cam- 
bium is the central feature of the process ; and much of the 



Ch. IV, 13] 



ECONOMICS OF STEMS 



209 



technique of grafting centers in making good contacts of 
cion and stock, and in holding the parts together until their 
permanent union is effected (Fig. 154). Grafting is mostly 
done in very early spring, when the tissues are resting, but 
are soon to become active. Later, as the tissues awaken, they 
knit together, the wound heals over, and thereafter they 
grow as one plant, without need of further attention, except 
that for a time care must 
-be taken to remove any 
shoots which spring up 
from the stock, for these, 
with their greater vigor, 
may draw all sap from 
the cion and cause it to 
perish. Ideally the pro- 
cess is simplest when cion 
and stock are the same 
diameter ; but very small 
twigs can readily be 
grafted upon very large 
stumps. Naturally an 
elaborate technique and , Fig. 154. — illustration of the method 

ot grafting. On the lett a cion of apple, 
great special knowledge prepared ; next, two cions inserted in a 

appertain to the subject. "^^^^^ °^ ^^^ "sht, the waxing of the 

. -in tissues to prevent desiccation and en- 

Grattmg is practiced for trance of Fungi. (From Bailey.) 

three principal reasons : 

First, and most important, it permits both the preserva- 
tion and the multiphcation of valuable kinds of plants which 
appear as bud sports, but which neither transmit their good 
qualities through seed, nor strike root from cuttings, and 
hence, except for grafting, would be lost. Bud sports, which 
are related to monstrosities (page 205), are individual branches 
which show in their development some striking difference 
from others on the same plant. Most of our best varieties 
of Apples, Pears, Oranges, and other fruits, have originated 
in this way, and are perpetuated only by grafting. Indeed, 




210 A TEXTBOOK OF BOTANY [Ch. IV, 13 

grafting may be defined from this point of view as a process 
of fitting a set of ready-made roots upon kinds of plants 
unable to make any of their own. 

Second, grafting can be used to produce certain desirable 
changes in minor qualities of the cion, though no essential 
features can thus be altered. An earlier or later time of 
blossoming or fruiting of a tree, a better adjustment to a 
particular soil or climate, advantageous dwarfing or enlarg- 
ing, resistance to root parasites, even in some small degree 
an improvement in color or size may be wrought in the cion 
by grafting on a suitable stock. All such features, however, 
seem to depend upon the sap, which of course is suppUed by 
the roots of the stock. The more essential characters are 
seated in the protoplasm, and remain unaltered by grafting, 
since the protoplasm, unhke the sap, does not pass from 
stock to cion, but remains separate in the two. 

Third, curious effects in plant form are obtainable by 
grafting, as when a dozen or more varieties of Cherries are 
made to grow on one tree, or bizarre constructions are pro- 
duced by the grafting upon one stock of many forms of Cacti, 
which happen to graft extraordinarily well. 

The older books upon horticulture frequently mention 
GRAFT-PI YBRiDS, of which the most famous is Cytisus Adami, 
produced by grafting between yellow-flowered and purple- 
flowered shrubs, and itself preserved by grafting. It shows 
diverse comminglings of yellow and purple in the flowers, 
but not an intermediate color. In a true hybrid, produced 
by the crossing through fertilization of two parents of dif- 
ferent races or species, the color is that of one parent or the 
other, or else has an intermediate shade, but is never a 
mosaic of the two colors, as in this plant. However, modern 
research has shown that Cytisus Adami is no h3^brid at all, 
but a mixture of the tissues of the two parents, such a com- 
bination being now called a chim^era. It has been found 
possible to produce these chimaeras artificially by so manip- 
ulating the grafting that a part of a bud of the cion unites 



Ch. IV, 13] 



ECONOMICS OF STEMS 



211 




with a part of a bud of the stock, in which case the resultant 
bud has the tissues of the two parents intermingled in diverse 
ways. Such chimaeras, accidentally produced, are not un- 
common in Oranges, or even in Apples, which sometimes have 
one segment of skin differing sharply in color or texture from 
the remainder. 

An important economic aspect of stem structure is in- 
volved in the new practice of tree surgery. In order to pre- 
serve valuable trees, it is now customary not only to prune 
away branches seriously affected by disease, 
but also to clean out cavities thus caused, 
and fill them with cement, in imitation of 
the methods successfully practiced by 
dentists with teeth. Experience, however, 
is hardly justifying earher expectations, for 
such cement-filled cavities, though seem- 
ingly at first satisfactory, often decay next 
the cement, which shrinks slightly in setting 
and allows sap to exude and Fungi to enter. 
Besides, the rigidity of the cement fits 
badly with the elasticity of trees which 
must sway in the wind, while its weight in 
some positions is a serious strain upon thin 
cylinders of wood. A promising, though 
rather expensive substitute, is a filling of 
wooden blocks set in an elastic, antiseptic material Uke tar. 
In other details tree surgery has made real progress, e.g. in 
the supporting of weak branches by chains and bolts, the 
former of which permit a free motion in the tree, while the 
latter prevents that choking of the bark which follows the 
use of encircling bands (Fig. 155). The subject is still in 
the developmental stage, on which account it offers a tempt- 
ing field to incompetent practitioners, and even impostors, 
against which type of '' tree-surgeons" the owner of trees 
must be upon guard. 



Fig. 155. — A 
good and a bad way 
to strengthen a 
weak crotch of a 
tree. Better yet, 
in many cases, is 
the use of a chain 
between two bolts 
instead of the single 
long bolt. (From 
Bailey.) 



CHAPTER V 
THE MORPHOLOGY AND PHYSIOLOGY OF ROOTS 

1. The Distinctive Features of Roots 

Roots are typically underground parts which spread 
through the soil and absorb therefrom the water and mineral 
matters needed by plants, while simultaneously providing 
a firm anchorage for the stems which rise in the air. Thus 
roots have a distinctive primary with a prominent secondary 
function. Though diverse in forms, and occasionally per- 
forming additional or substitute functions, they are less 
multiform in these features than leaves or stems, no doubt 
because of the more homogeneous environment under which 
they dwell. 

Typical soil roots extend from the base of the stem, and 
continuously radiate, branch, and taper down to a fibrous 
size. Taking all angles from vertical to horizontal, they form 
collectively a mass suggestive of some shoots, but inverted 
(Fig. 156). Unlike shoots, however, they are rarely sym- 
metrical, because mechanical irregularities in the ground, 
and self-adjustments to the uneven distribution of water, 
air, and mineral salts, greatly alter their shapes, making 
actual root systems extremely irregular. The radiate form, 
so distinctive of soil roots, enables them to reach a large 
volume of soil, while also providing the best anchorage 
against the all-sided strains to which stems are exposed ; but 
there also occur cases in which a single main root continues 
the stem vertically downward, the lateral roots being very 
much smaller. Such a tap root (Fig. 157) is rare in trees 
but common in herbs, as familiar in Dandelion and others, 

212 



Ch. V, 1] 



FEATURES OF ROOTS 



213 



where often it is used for storage of food. That the mass 
of roots keeps towards the surface, especially in the largest 
plants, is due in part to the need for aeration, and in part to 
the increasing hardness of the soil with greater depth. 

In size, roots bear close relations to shoots, for it is clear 
that the shoot takes the lead, so to speak, in determining the 
form and habits of the 
plant, and secondarily 
produces a corresponding 
quantity of roots. No 
matter what the size at 
the trunk, all roots end in 
the delicate white tips de- 
voted to absorption and 
growth ; and in correlation 
with this uniform function, 
performed under compara- 
tively uniform conditions, 
the tips of typical soil roots 
are not far from one size. 

In texture, roots vary 
from woody-hard in trees 
(the wood, indeed, of roots 
being often harder and 
more compact than that of 
the stems) down to the 
softness of meristem in 
growing tips. The fibrous 
parts are tenaciously tough, 
— a quality which is evi- 
dently connected with the fact that the anchorage function 
of the roots falls largely on the fibers. 

In color, roots are white at their growing tips, that being 
the natural color of meristematic tissue. Farther back they 
are brown, from the development of protective coi'k; and 
in older parts they are very dark from the action of the 




Fig. 



156. — A typical root system, 
Corn. (From Bailey.) 



of 



214 



A TEXTBOOK OF BOTANY 



[Ch. V, 1 



soil on the bark. Sometimes, when exposed to the Hght, 
young roots will turn red, apparently through formation of 
erythrophyll, which may have any of the meanings already 
explained for that substance (page 88). 

In duration, roots conform to the plants which produce 
them, being annual, biennial, or perennial, and either herba- 
ceous or woody. 
Unhke shoots, 
however, roots 
drop no parts, 
for the gromng 
tips develop 
without break 
into the older 
and thicker, 
and finally the 
woody parts. 

Roots are pro- 
duced from 
stems, most 
commonly and 
tjrpically from 
the lower end of 
the first stem 
formed by the 
embryo plant ; 
but sometimes 
they develop 
from other parts, particularly from the nodes where these 
happen to touch the ground. Further, many kinds of 
plants, like the common ''Geraniums," which do not 
naturally produce roots from their stems, can be made to 
do so from slips or cuttings, though this is impossible with 
most kinds. Sometimes, though rarely, roots produce stems, 
as in Locusts and Apple trees, which send up suckers from 
their roots at a distance from the trunks. 




Fig. 157. 



A typical tap root, of Dandelion. 
(From Bailey.) 



Ch. V, 2] 



STRUCTURE OF ROOTS 



215 



True soil roots are found only in the Flowering Plants and 
Ferns. The lower land plants (the Bryophytes, or Moss 
plants) have substitutes in large hair-like rhizoids. The 
Algae need no roots, since they absorb through their whole 
bodies, though the Rockweeds have attachment organs, 
somewhat hke roots in aspect. In the Fungi no roots occur, 
although their slender absorbing mycelial threads (page 84) 
possess certain characteristics of root hairs. 

While soil roots are primarily organs of absorption and 
anchorage, they also perform other functions, becoming 
storage organs, spines, climbing organs, and even foliage, as 
will presently be noted. 

2. The Structure of Roots 

The principal features of root structure can be seen very 
well in the root system of some garden herb or house plant 
carefully hfted and washed free 
of adherent soil. Observation of 
such material shows that the 
entire root system of a plant is 
continuous, without any trace of 
such nodes as occur in the stem. 
Each part is typically cylindrical 
though often forced by the soil to 
other shapes. The branching is 
very irregular, in marked con- 
trast to the phyllotactic sym- 
metry of the shoot, but answering 
to the composition of the soil; 
but in some seedlings the first side 

roots appear in vertical rows corresponding to the fibro- 
vascular bundles which enter the roots from the stem, — e.g. 
in Bean seedlings four such rows occur. All new branches of 
roots originate deep in the tissues, in contact with the fibro- 
vascular bundles, whence they make their way out through 
the overlying tissues, partly by the solvent action of diges- 




FiG. 158. — Cross section of 
the fibrous part of a young root 
of a Bean, Phaseolus multiflorus. 
(From Sachs.) 



216 



A TEXTBOOK OF BOTANY 



[Ch. V, 2 




live enzymes, and partly by mechanical rupture, as a later 
picture illustrates (Fig. 164). This method of origin contrasts 
greatly with that of leaves, which arise as surface swellings 
in the bud, while the origin of branches is intermediate in 

nature. The vein, or 
fibro-vascular, system 
of roots is in perfect 
continuity with the 
systems in stems and 
leaves. The separate 
fibro-vascular bundles 
of young roots, clearly 
visible in sections by 
aid of a hand lens, 
differ little from those 
of the stem, although 
the fibro-vascular sys- 
tem of roots as a whole 
is more strongly con- 
densed towards the 
center, often obliterat- 
ing the pith (Fig. 158). 
Thus, while stems ap- 
proximate, as we have 
seen (page 118), to the 
hollow-column princi- 
ple of construction, 
roots are built rather 
on the plan of cords or 
cables. The difference 



Fig. 159. 



A typical root tip, of Radish ; 
magnified. 



is obviously correlated with the different kinds of strains 
the two parts have to bear; for, while stems are exposed 
to great lateral strains from the winds (and, on the non- 
vertical parts, from their weight), against which the hollow 
column is most effective, the roots are exposed only to 
pulling strains, in resistance to which the solid cable is best. 



Ch. V, 2] 



STRUCTURE OF ROOTS 



217 



The most highlj^ developed roots, those of our exogenous 
trees, show three distinct though intergradient parts, — 
viz. the slender white tips, the elongated fibers, and the 
thick woody parts. 

The tips, best seen in material grown for the purpose in 
moist air or moss, show really four parts (Fig. 159). Firsts 




Fig. 160. — The root-hair zones and growth zones in some common roots ; 
X X- From the left, Pea, Radish, Corn, Lupine, and, below, Oats. The 
seeds were germinated in moss behind sloping glass plates. 



the actual end of the root consists of a root cap, formed 
from behind by the very deUcate growth tissue, to which 
it acts as a protection in the advance of the root through 
the soil. Second, just behind the root cap lies a yel- 
lowish spot, which marks the growing point, the place 



218 



A TEXTBOOK OF BOTANY 



[Ch. V, 2 



of formation of all new cells by which the root increases in 
length, the color being that of the abundant Hving proto- 
plasm showing through the transparent walls. Third, 
just behind the growing point lies a short smooth zone, which, 
though little prominent, has yet this great importance, that 
it is the GROWTH zone, or place of enlargement to full size 
of the new cells formed in the growing 
point. The growth of the root in length is 
wholly confined to this zone (though new 
cells cause an increase in diameter farther 
back), in great contrast to the conditions 
in stems, where the growth occurs through 
several expanding internodes (Figs. 112, 
114). Fourth, just behind the growth zone 
^ comes another, differing greatly in length in 
different plants and under different condi- 
tions, the ROOT HAIR ZONE (Fig. 160). The 
ROOT HAIRS thereon show remarkably well, 
especially through a lens, in roots germi- 
nated from seeds in moist air, though they 
have no such regularity of shape in the soil 
(Figs. 161-2). In the former material the 
hairs radiate very evenly outward, forming 
collectively a sort of nimbus along the root ; 
and they are obviously forming anew in 
front, going each through its grand period, 
Fig. 161. — Rad- and dying behind. Thus the zone moves 
ish seedlings grown along as a wholc iust behind the advancing 

in moist air and m . ,. i i • • ^^ 

soil. (From Sachs.) tip. The function of the hairs is well 
known ; they provide the great surface 
necessary for the absorption of the water when it is scant 
in the soil. They pass this water through the cortex to 
the ducts, which extend all the way from this zone to the 
leaves. We can now see a reason why the entire growth 
of the root in length takes place in advance of this zone, 
for any growth behind the hairs would obviously tear them 





Fig. 162. — Longitudinal sections through a root of Corn, at the growing 
point, growth zone, and hair zone ; highly magnified. 

The scale of the drawing is not large enough to permit the representation 
of all of the details mentioned in the text. 



220 A TEXTBOOK OF BOTANY [Ch. V, 3 

from the root. In cross sections one can see the fibro-vascular 
bundles lying so closely towards the center as greatly to re- 
strict the area of the pith, or even to obliterate it altogether, 
though there is always a relatively thick cortex (Fig. 163). 

The tips of the soil roots of different plants are far more 
uniform in size, and especially in diameter, than are the 
leaves and young stems, — of course because of the more 
uniform environment presented by the soil. Exact measure- 
ments show that in ordinary plants, the roots at the growth 
zone vary in diameter from .3 to 1.07 mm. with a mean at 
.67 mm., while the side roots vary from .19 to .79 with a mean 
at .53, giving a conventional constant of .6 mm. for the diam- 
eter of root tips in general. This size bears without doubt a 
relation to the conditions of water absorption by the roots, 
analogous to the relation of leaf -thickness to light (page 
33), though the precise factors have not yet been deter- 
mined. 

Backwards the young white tips merge gradually into the 
familiar brown, fibrous roots. Cross sections thereof show 
the formation of a corky bark, the beginning of a secondary 
growth in thickness of the bundles (in exogenous kinds), 
and a general aspect of toughening of the tissues; for this 
is the part of the root which seems to take much of the strain 
of the anchorage function. 

In herbaceous plants, as a rule, the roots remain fibrous, 
but in shrubs and trees they grow continuously in thickness 
by the activity of the cambium, quite after the manner of 
the stem. Thus they develop a distinct bark and wood, 
with annual rings, medullary rays, and other features already 
famihar in stems. Indeed, except for their underground 
position, such roots are practically stems. 

3. The Cellular Anatomy of Roots 

As with other plant organs, the cellular anatomy of roots 
is linked so closely with their functions that the two recipro- 
cally throw light upon one another. 



Ch. V, 3] ANATOMY OF ROOTS 221 

A very thin section cut longitudinally through the tip 
of an ordinary root, e.g. of Corn, presents under the micro- 
scope the aspect here pictured (Fig. 162). Close to the coni- 
cal end stands out the growing 'point, distinguished by its 
many small, densely-packed cells, which are squarish in 
section, thin-walled, and filled with the all-important 
protoplasm. This is the place of cell-formation for the en- 
tire tip of the root, the new cells being made by division 
from a small central group, after which they absorb nourish- 
ment and enlarge to the original size. In front these new 
cells are constantly forming the root cay, becoming larger 
and empty near the outside, where they are continuously 
abraded away by the forcible passage of the root through the 
soil. Backwards, in the growth zone, the cells hold, the reg- 
ular ranking in which they are formed} but grow rapidly 
larger, especially in length, while keeping their thin walls, 
to which the protoplasm comes soon to form only a fining. 
Each individual cell, in fact, immediately after its formation, 
goes through a grand period of enlargement (page 156), soon 
reaching its maximum size : and this explains how the 
growth zone follows so closely behind the growing point. 
Here also can be seen the beginning of the cellular differentia- 
tion of the fibro-vascular bundles, while the intercellular 
aeration system also is plain, though it does not appear in 
our drawing. Backward the growth zone merges impercep- 
tibly into the hair zone. The hairs originate as sfight swelfings 
from the outer walls, and grow rapidly longer until they 
attain the famifiar tube form. In this zone appear also the 
striking fine spirals of the ducts, of which the mode of for- 
mation is clearly apparent in good sections. The end walls 
in a long fine of superposed cylindrical cells break down, 
under action of digestive enzymes, while simultaneously 
the spirals begin to appear as local thickenings of the 
walls. 

These sections show further that the outermost layer 
of cells of the root possesses no breaks or openings of any 



222 



A TEXTBOOK OF BOTANY 



ICh. V, 3 



description, the walls being everywhere continuous. This 
absence of stomata is perfectly explained by the habits of 

roots, which have no 
chlorophyll and need no 
cutinized epidermis. The 
oxygen used in the respira- 
tion of the roots passes in 
solution directly through 
these walls, which are 
uncutinized. 

Cross sections bring out 
several additional features 
(Fig. 163). Here can be 
seen more distinctly the 
innermost layer of the 
thick cortex, called the 

ENDODERMIS (Fig. 163), 

the exact morphological 
equivalent of the starch 
sheath of stems (page 130). 
In the roots, however, the 
walls of this layer are 
partially cutinized, espe- 
cially on the radial parts, 
for reasons not yet under- 
stood. Also there appears 
a notable difference in the 
arrangement of the young 
fibro- vascular bundles as 
compared with the stem. 
The xylem, recognizable 
by the very large size 
of the ducts, and the 
phloem, distinguished by 
the smaller angular form 
of the sieve tubes, do not 




Fig. 163. — Cross section of a root of 
a Bean, Vicia Faba, just behind the hair 
zone; X 80. 

The four strands of xylem meet in the 
center, obliterating the pith, while the 
strands of phloem stand separately be- 
tween the arms of the cross thus formed. 
Between xylem and phloem can be seen 
the developing cambium, which presently 
begins to form new xylem inside of the 
phloem, thus originating bundles of the 
ordinary stem type. Surrounding the 
fibro-vascular system is the endodermis, 
and outside thereof the very wide cortex. 
(Fibro-vascular system after L. Kny, the 
remainder drawn from nature.) 



Ch. V, 3] 



ANATOMY OF ROOTS 



223 



stand in-and-out from one another but alternately, or in 
different radii. This arrangement, found in all roots, has 
been viewed as adaptive, in removing the phloem out of the 
path of transfer of the water from root hairs to ducts ; and 
support is given this supposition by the fact that immedi- 
ately behind the hair zone the arrangement is abandoned, 
for the new xylem 
and phloem made by 
the developing cam- 
bium stand in-and- 
out from one another 
as in stems. The 
method by which 
the cambium makes 
the transition from the 
one arrangement to 
the other is easily un- 
derstood by aid of 
the figure. Endog- 
enous roots do not, 
of course, form a cam- 
bium, but have sepa- 
rated closed bundles 
as in their stems. 

Just behind the hair 
zone the cambium 
begins the secondary 
increase in thickness, 
by addition of xylem 

from its inner and phloem from its outer face, precisely as with 
stems. Farther back along the root, one can see here and 
there in cross sections the mode of formation of the new side 
roots, which come from the fibro-vascular bundles, and make 
their way to the surface, as already described (Fig. 164). 

In the thick woody parts of the roots of shrubs and trees 
the cellular anatomy is nowise essentially different from 




Fig. 164. — Longitudinal section of a root 
of Corn, showing the origin of a side root ; 
highly magnified. 

The side root develops in contact with a 
fibro-vascular bundle, and "dissolves" its way 
out, by action of enzymes, to the surface. 



224 



A TEXTBOOK OF BOTANY 



[Ch. V, 4 



stems. Indeed, except for the relics of their early root 
anatomy deeply bmied within their tissues, and their some- 
what greater compactness of texture, such roots are stems, 
both structurally and physiologically, despite their under- 
ground position. 

4. The Absorption of Water, and Other Functions 
OF Roots 

Typical roots perform one primary function, viz. absorp- 
tion of water and mineral matters ; one secondary function, 

viz. anchorage for 
the stem ; and one 
or two minor func- 
tions supplemen- 
tary to these. 

Water is the 
most necessary of 
all the materials 
absorbed by plants, 
in which it is used 
for six purposes. 
First, it forms an 
essential constit- 
uent of the photo- 
synthetic food 
(page 21). Second, 
it constitutes a 
large proportion of 
the composition of 
plants, amounting 
(as shown by com- 
parative weighings 
of fresh and dried material) to more than 90 per cent in 
most herbaceous parts. Third, it holds the soft parts tensely 
spread by high sap pressure within the cells. Fourth, it is 
a necessary solvent for the many chemical reactions in 




Fig. 165. — Typical root hair, and cortical cells, 
in a longitudinal section of Radish. (After a wall 
diagram by Frank and Tschirch.) 



Ch. V, 4] 



ABSORPTION BY ROOTS 



225 



progress in plants, such reactions rarely occurring except 
in solution. Fifth, it provides a medium of transport, 
in form of solution, for substances through the plant. Sixth, 
it is needed to compensate the incessant loss by transpira- 
tion. These are the 
reasons why plants 
must have plenty of 
water. 

The water used by 
ordinary plants is 
wholly absorbed 
through their roots, 
and none is taken 
through leaves or 
stems. Further, the 
actual absorption is 
kno^Ti to take place 
in the young parts of 
roots, and mainly 
through the root hairs. 
The hairs are thus 
effective, not through 
any special power de- 
nied to other cells of 
the young root, but 
simply through the 
great surface they 
spread. It is because 
these hairs, tightly 
adherent to the soil, 
are mostly torn away t? ^aa k ^ t ^ i^ u- 

^ _ *^ -b iG. 166, — A plan of a root as an absorbing 

when roots are lifted mechanism, arranged as in Figs. 11 and 105, 

from the soil that ^^^^ similar signs for water, protoplasm, and 

' ^ sugar. At the tip the growing point ; at the 

plants commonly wilt left, pith ; a duct ; two rows of cortex ; the root 

on transplanting, and ^^^f • , ^°*^ ^^^^ ^^^l ^A^^ ^°"*^f ^°^*^^^ 

^ ^^ protoplasm and sugar, but the duct contains 

recover only after neither. 




226 



A TEXTBOOK OF BOTANY 



[Ch. V, 4 



new tips and hairs have again made connection with the 

water supply. 

Each root hair is a cell, 
possessing a cellulose wall 
lined by living protoplasm 
(Fig. 165) and a sap con- 
taining various substances, 
especially sugar, in solution. 
The hairs are in close con- 
tact with particles of soil, 
and bathed in the surround- 
ing water (Fig. 169). In 
the root they are in con- 
tact with the cortical cells, 
which likewise have cellu- 
lose walls, protoplasmic 
hnings, and sugar-contain- 
ing sap ; and the cortical 
cells in turn are in contact 
with the ducts which have 
no protoplasmic linings. A 
typical example of this 
absorbing system is shown 
by an earher picture (Fig. 
159), while its mechanical 
construction is illustrated 
by our diagrammatic Fig- 
ure 166. 

The water in the ducts, 
while sometimes containing 
sugar and the hke, is ordi- 
narily nothing other than 

Fig. 167. —a pressure gauge at- -i ,„^|p„ „^|U onmP min- 
tached to a root for the measurement ^^^^ Water, Wltn SOme mm- 

of sap-pressure ; X j. eral matters in solution. 

The rise of the mercury in the long Fiirthprmnrp this water is 
tube above the level in the reservoir bulb ^ Urtnermore, tniS water IS 
gives the sap-pressure in "atmospheres." forced intO the ductS by 




Ch. V, 4] ABSORPTION BY ROOTS 227 

the cortical cells under considerable pressure, as manifest to 
the eye when a suitable pressure-gauge is attached to the 
cut stump of an active plant (Fig. 167). Thus tested, potted 
plants will often show a root pressure, i.e. a pressure of water 
in the ducts, sufficient to raise water over thirty feet, while 
some trees show two or three times as much. This pressure 
is not enough to raise water to the tops of the tallest trees, 
but it does give the sap a good start up the stem, after 
which it is lifted to the leaves by the forces we have earlier 
considered (page 147). This root pressure, however, is the 
source of the ''bleeding" of broken or pruned stems in the 
spring, and also of guttation. 

What then is the nature of the power by which the root 
hairs absorb water and give it so forcible a push up the 
stem ? Evidently the water absorbed by the hairs and passed 
through the cortical cells must pass through walls and proto- 
plasm, which are membranes, and through the cell solutions, 
which, for simplicity, we can consider as solutions of sugar, 
their most prominent constituent. Such absorption is 
known in physics under the name osmosis, and so important 
a part does osmosis play, not only in absorption of water, 
but also in other physiological phenomena, that the student 
should not fail to make its acquaintance through experiment. 
Any simple device in which a membrane, e.g. a piece of 
parchment, separates a sugar solution from water, will 
serve the purpose ; but a specially convenient arrangement 
is represented in the osmoscope shown in Figure 168. When 
a solution (molasses is a very convenient solution of sugar) 
is placed in the parchment tube, which then is immersed 
in water, the solution will rise in the vertical tube at a 
distinctly visible rate. If instead of water a solution 
weaker than that in the parchment tube be used, the result 
is the same, though the rise is slower. If the water be 
placed inside and the solution outside, there is no rise, but the 
tube soon empties, shrinks, and collapses. These phenom- 
ena are typical, and the osmotic process may be generalized 



228 



A TEXTBOOK OF BOTANY 



[Ch. V, 4 



thus, — whenever a solution and water, or two solutions of 
different strengths, are separated hy a membrane which they 

can wet, there is always a 
movement from the weaker to 
the stronger at a rate propor- 
tional to the difference in 
strength. 

In the foregoing experiment, 
though the solution rises in 
the tube, some also escapes 
into the water, as shown by 
its color when molasses is 
used. From the root hairs, 
however, no sugar escapes to 
the soil. When we seek a 
structural reason for this dif- 
ference, we find that the root 
hair possesses a protoplasmic 
lining, which has no counter- 
part in the tube. It is, how- 
ever, entirely possible to make 
up from certain common 
chemicals, and supply to the 
parchment tube, a lining which 
in this respect acts like the 
protoplasm, viz. it permits 
water to enter, but no sugar to 
pass out; and such ^'artificial 
cells" are often constructed in 
botanical laboratories. Thus 
Fig. 168 -An osmoscope for ^^ ^^^ ^^^^ membranes exist 

the demonstration of osmosis ; X 5. 

The larger jar contains water, the which permit both water and 

tube inside is parchment paper^ and ^ to paSS (PERMEABLE 

the dark liquid is molasses. When ^ f \ 

this liquid has risen to the top of membranes), while others per- 

the open tube it can be dropped -^ j ^ ^ ^^^^^_ 

back to level by opening the stop- -^ . 

cock of the reservoir-funnel. PERMEABLE membranes) . This 




\ 



Ch. V, 4] ABSORPTION BY ROOTS 229 

difference is vastly important in both plant and animal 
physiology. 

It is perfectly clear that the water passes osmotically 
into the root hair cells, and thence to the cortical cells, 
which have solutions as strong as the hairs, or stronger. In 
small simple plants, especially the Moss plants, the water 
moves thus from cell to cell, throughout the plant. But 
where ducts are present, as in all of the Flowering Plants and 
Ferns, the water passes from the innermost cortical cells 
into those ducts, and does so as pure water, and not as a 
sugar solution. Why does water leave the cortical cells, when 
it enters the similarly constructed hair cells ? In a physical 
machine it would not do so ; the cortical cells would absorb 
water from the ducts, instead of giving it out to them, 
precisely as in case of the hairs and the soil. Herein we 
face a. still unsolved problem of plant physiology. Several 
methods are imaginable, though none have been proven; 
but there is little doubt that the explanation will be found 
in some simple chemical or physical change controlled by 
the living protoplasm. Presumably the method is dependent 
on the relatively great thickness of the cortex in all ab- 
sorbing roots; and it may prove that each cortical cell 
contributes a little towards breaking the osmotic hold 
on the water, the cooperation of many being therefore 
essential. 

In the experiment described a few pages earher the solu- 
tion was free to rise. What happens when the tube is 
closed ? In this case pressure always develops, first stretch- 
ing, and then bursting the cup, unless very strong ; and if a 
suitable gauge be attached, the pressure can be measured. 
The results are surprising, for with cells specially built for 
great strength, and the use of strong sugar solutions, osmotic 
pressures have actually been measured in excess of 24 atmos- 
pheres, that is, 360 pounds to the square inch, which is more 
than the pressure in most steam boilers ; and we know that 
greater pressures occur. In cells of the higher plants the 



230 A TEXTBOOK OF BOTANY [Ch. V, 4 

pressures are much lower than this, usually not more than 
10 to 20 atmospheres, though in the lower plants, especially 
some Molds and Bacteria, there is reason to believe that the 
pressures rise often far above the 24 atmospheres just men- 
tioned. 

Such striking and important phenomena as osmotic ab- 
sorption and pressure demand explanation, which, however, 
cannot yet be given with certainty. A close quantitative 
relation exists between osmotic pressure and gas pressure, 
on which account some investigators have considered them 
identical, holding that a substance in solution is virtually 
in the state of a compressed gas, and exerts a gaseous pres- 
sure. Others, however, maintain that nothing more is involved 
than the adhesive affinity of the sugar, or other dissolved 
substance, for the water, — the substance confined within 
the membrane drawing and holding the water which 
can pass the membrane freely. The most probable explana- 
tion makes it a result of the checked diffusive power of the 
dissolved substance, which cannot escape through the mem- 
brane, though the water can enter. As to the passage of 
water, and (in case of some membranes) dissolved substances, 
through membranes which seem perfectly solid, that clearly 
occurs between the ultimate structural units of the mem- 
brane, whether molecules or other units. But the subject is 
too recondite for further discussion at this place. 

The mineral matters needed by plants are compounds 
which contain the following seven elements, — viz. ni- 
trogen (which plants cannot absorb from its uncombined 
state in the air, and therefore must obtain from compounds 
in the soil) ; sulphur and phosphorus, integral constituents 
of proteins, and therefore of hving protoplasm ; potassium, 
needed for incidental processes in connection with the forma- 
tion of carbohydrates ; calcium, a neutrafizer of injurious 
substances ; magnesium, an integral constituent of chloro- 
phyll, with iron, incidentally necessary in some way to the 
formation thereof. These elements all occur in mineral 



Ch. V, 4] ABSORPTION BY ROOTS 231 

salts dissolved in the soil water with which they are absorbed 
into the plant. Though other mineral matters are also 
absorbed, only those which contain these elements are 
invariably essential ; and if we add the three elements, car- 
bon, hydrogen, and oxygen, we have a list of ten elements, 
indispensable to the life of the higher plants. 

Not all of the mineral salts dissolved in the soil water are 
absorbed equally by plants, or in the same proportions by 
different plants; but in how far this seeming ''selective 
power" of roots is merely incidental to their physical and 
chemical constitution, and in how far it acts adaptively to 
the needs of the plant, is still uncertain. Probably, as in 
most such phenomena, something of both is involved. 

Such is the method of the primary function of roots, that 
of absorption. The second function, anchorage of the plant 
in the ground, is chiefly mechanical and comparatively simple. 
Against the lateral strains upon stems from the action of 
winds, a suitable resistance is provided in the radiating 
disposition of the roots, with their tough cord, or cable, 
type of construction. There is good reason to suppose that 
roots subjected to the greatest strains may become thicker 
and tougher in adaptive self -adjustment thereto, in the 
very same way that our own muscles grow stronger through 
exercise. 

In addition to the two functions which roots perform as 
their peculiar contribution to the economy of the plant as a 
whole, they have also certain others essential to their own 
individual well-being, — notably respiration and growth. 
Respiration in roots has precisely the same method and 
meaning as in other parts of the plant (page 165). Roots, 
accordingly, require air, and this need has a dominating in- 
fluence upon many features of their habits and structure. 
In plants which live in bogs, marshes, swamps, and other 
places of standing water, the air is usually transferred to the 
roots from the leaves along the intercellular air system, 
which in such cases is specially developed. By ordinary 



232 A TEXTBOOK OF BOTANY [Ch. V, 5 

roots, however, air is absorbed from the supply contained in 
the porous soil. Roots have no stomata, or other openings 
in their equivalent for an epidermis ; but the air in the soil 
becomes dissolved in the water, and goes in solution through 
the saturated walls into the cells of the root, from which it 
passes to the air spaces, where it re-collects in the gaseous 
form and thus reaches other parts of the root. The carbon 
dioxide produced in respiration diffuses out to the soil by 
exactly the reverse process. It is because of self -adjustment 
to a more abundant air supply (aerotropism) that most of the 
roots of great plants do not commonly penetrate far into the 
ground, but keep close to the surface. This is also the reason 
why trees commonly die when their roots are deeply buried, 
as sometimes happens in grading around new buildings. 
Protection of roots against desiccation, the ever present 
danger to leaves and stems, is effected incidentally by their 
position within the damp ground. Thus it is possible for 
the young tips to dispense with a cutinized epidermis, which 
would be inconsistent with their absorptive function. The 
older, roots develop a bark, but it is thin as compared with 
that of the stems. 

5. Osmotic Pkocesses in Plants 

The absorption of water by roots is only one of several 
important plant processes in which osmosis has part. It is 
important to recall that osmosis is a physical process, though 
living protoplasm may regulate the conditions of its opera- 
tion : that it occurs wherever in Nature two solutions of 
different strengths are separated by a membrane they can 
wet : that in such case there is always a movement from the 
weaker to the stronger solution : that the movement in- 
volves both solvent and dissolved substance in case of per- 
meable membranes, but the solvent only in the semi-permeable 
kind : that the stronger solution will swell and rise if free, but 
when confined will develop pressure which can become very 
great. Also its rate is directly proportional to temperature. 



Ch. V, 5] OSMOTIC PROCESSES 233 

The most striking utilization of osmotic pressure by plants 
consists in the maintenance of the form and rigidity in leaves, 
young stems, flowers, and other soft herbaceous parts. So 
small is the percentage of solid matter in such tissues (not 
over 10 per cent, with 90 per cent of water), and so thin and 
flexible the cell walls, that they cannot alone sustain their 
own weight, as shown by their collapse in wilting. These 
herbaceous parts are held tensely stretched and outspread 
in their characteristic forms by the osmotic pressure of their 
sugar-containing sap inside the thin-walled cells, the needful 
water being supplied from the ducts. That herbaceous tissues 
owe their stiffness to osmotic turgescence may be proven 
conclusively by the simple experiment of immersing them 
in a solution having a greater osmotic strength than the sap, 
in which case of course an osmotic movement out of the 
cells will take place. The result is always a collapse of the 
tissues even more striking than wilting produces. It is true 
the experiment works badly with leaves and stems, because 
the waterproof epidermis almost prevents osmotic move- 
ment; but the effect is perfect in parts without epidermis, 
such as strips cut from Potatoes or Beets. These become 
soft and flexible after only a few minutes' immersion in 
strong sugar or salt solution, although comparison strips are 
rendered stiffer and harder than ever by immersion in 
pure water. Not only do such tissues become flaccid by 
wilting or immersion in strong solutions, but they also 
shrink in area, thus proving that the tense cells are held 
actually stretched by the osmotic pressure within them. 
The stiffness which pressure of water can give is familiar 
also in fire-hose. 

Equally important is the role of osmotic pressure in growth, 
for it supplies the mechanical power whereby the newly formed 
cells expand in size, often against much resistance of the 
overlying tissues. The young cells osmotically absorb 
water, and the resultant pressure stretches the wall, in 
which new cellulose is continuously laid down by the proto- 



234 A TEXTBOOK OF BOTANY [Ch. V, 5 

plasm until the cell is full-grown. By use of the same power 
roots force' and enlarge for themselves passages through 
hard soil, even prying aside stones in the process ; and by 
the same power they disrupt masonry and lift curbstones in 
streets. So essential is osmotic pressure to growth, and 
hence so indispensable is adequate water to growing plants, 
that any marked scarcity of water, or rapid removal thereof 
from the plant, always checks its growth. This is why the 
growth rate of a plant always falls, other things being equal, 
when transpiration becomes active, and vice versa : why 
plants tend to grow faster at night than in daytime : and 
why growth usually is checked with the sunrise. 

The question must now occur to the student, whether 
osmotic pressure can ever become so great as to strain if not 
burst the plant cells. This does in fact sometimes happen. 
Thus some fruits, notably Plums, in warm moist weather 
occasionally burst, from this cause, on the trees. In Tomato 
plants, watery blisters are sometimes formed osmotically, 
producing a kind of ''physiological disease" called Oedema. 
Most kinds of pollen (the small yellow grains producing the 
male cells in flowers), when placed in water, swell and burst, 
of course to their destruction. This result would be caused 
by the rain were it not that in most flowers the pollen is 
well protected therefrom by its position, or other arrangements, 
as will later be noted (page 295). A case of protective ad- 
justment against excessive osmotic pressure seems involved 
in the starch formation in leaves. In green leaves in the 
light, as the student will recall, the appearance of starch is 
always preceded by the formation of sugar, the starch being 
formed only after a certain concentration of the sugar lias 
been reached. The starch, however, is always re-converted 
to grape sugar when the concentration again falls, and thus 
is translocated into the stem. Now this seemingly useless 
formation of starch finds an explanation in the fact that while 
grape sugar exerts osmotic pressure, starch exerts none. 
The conditions are all consistent with the supposition that 



Ch. V, 5] OSMOTIC PROCESSES 235 

as the concentration of the photosynthetically-formed sugar 
approaches a quantity which might exert injurious action on 
the cell, the surplus is converted automatically into starch. 
The insoluble proteins found abundantly in sieve-tubes have 
presumably a like explanation, as has the cane sugar found 
in some leaves intermingled with grape sugar, for cane sugar, 
weight for weight, exerts only about half the osmotic pressure 
of grape sugar. In this latter fact, indeed, is probably found 
the reason why cane sugar is so much more common a storage 
form than grape sugar, as Sugar Cane, the Maple tree, and 
Sugar Beets illustrate. The fact that such changes, easily 
effected by plants, can produce so great a difference in osmotic 
properties may help to explain how the water is released 
from the cortical cells of the roots (page 229) . 

A striking and important feature of osmotic phenomena 
in plants is this, — that the living protoplasm lining the 
cells can act either as a permeable membrane, permitting both 
water and dissolved substances to pass, or as a semi-perme- 
able membrane, permitting only water to pass, or can act 
at one time as one kind and at another as the other. These 
various movements, complicated by the nature of the 
many chemical substances present, and by special phenom- 
ena of diffusion, solution, imbibition, and like molecular 
processes, explain, on a purely physical basis, many of the 
most important phenomena in plant physiology. 

Aside from the living plant, many osmotic phenomena in 
plant tissues are familiar in our daily experience. When 
shrunken currants or raisins are immersed in water, es- 
pecially if heated in cooking, they swell tensely, — for there 
is sugar in their cells. Berries cooked with little sugar swell 
and burst (though expanding air confined in the tissues also 
plays a part) ; but cooked with much sugar, as in preserving, 
they collapse. Dry sugar placed on fresh strawberries soon 
becomes a sirup, while the berries soften and shrink. The 
osmotic explanations are all obvious. We place cucumbers 
and celery in cold water to crisp them, that is to make their 



236 A TEXTBOOK OP BOTAISIY [Ch. V, 5 

soft cells more tense and explosive ; but warm water is not 
used because it tends to fill the air spaces and thus deaden 
the explosions. Sugar and salt are effective preservatives of 
fruits and meats, though not in themselves deadly to the 
Hving organisms (germs) which cause decay; and the fact 
that those substances must be used in great strength sug- 
gests the explanation, that they inhibit the activity of the 
germs by osmotically robbing them of water. Beans or 
rice are cooked more quickly and perfectly if not salted until 
nearly done, and indeed if placed in water too strongly 
salted at the start may refuse to swell at all. The sensation 
of thirst which follows the eating of much sugar or salt has 
apparently this basis, that those substances withdraw water 
from the stomach, thus causing the thirst sensation. The 
student will be able to cite other examples of osmotic phe- 
nomena in daily experience. 

' Closely connected with osmosis, of which it is part, is 
DIFFUSION. When the molecules of a substance are re- 
moved beyond the range of one another's cohesive attrac- 
tion, as in a gas or a solution, they acquire an active 
back-and-forth motion from the kinetic energy of the 
heat waves reaching them from the surroundings. Thereby 
they strike and rebound from one another, and hence are 
worked outward, exerting pressure if confined, but spreading 
indefinitely if not. Accordingly by diffusion any substance 
as a gas or a solution always tends to work away from places of 
greater to places of lesser concentration, and away from a place 
where it is being produced, and towards a place where it is 
being absorbed, each substance diffusing in general as though it 
alone were concerned. Familiar phenomena of gaseous dif- 
fusion occur in the spread of odors through a house, of floral 
fragrance through gardens, and of smoke through the air; 
while solution diffusion is illustrated by the spread of ink 
or sugar through water. This is the way that carbon dioxide, 
in photosynthesis, passes from the great reservoir of that gas, 
the atmosphere, through the stomata and along the air- 



Ch. V, 6] STRUCTURE OF SOILS 237 

passages to the places of use in the chlorenchyma ; and the 
way in which the oxygen as released passes outward along 
the same passages and stomata. It is also the method by 
which sugar and proteins made in chlorenchyma cells pass 
from cell to cell until the veins are reached, and then along 
sieve tubes and sheath cells to places of storage or use in 
stems or roots. It is probably also the ultimate source of 
osmotic pressure, which is diffusion pressure (page 230). No 
matter, however, what the details may be, the energy of 
diffusion is in all cases the same, — heat from surroundings. 
Two other physical processes important in plant physi- 
ology must here receive mention. Cell walls, if of cellulose 
or lignified but not if cutinized, absorb water forcibly by 
IMBIBITION, which rests fundamentally upon adhesive affinity 
between wall and water. A familiar manifestation occurs 
in the warping of boards, which occurs as result of access of 
water from one side, or its removal from one side by heat. 
Likewise certain dry cell walls can absorb water as vapor 
from the air, even producing forcible swelling and move- 
ments of the structures concerned ; and such hygroscopic 
phenomena occur in connection with the dissemination of 
seeds, and elsewhere, as will later be noted. The other pro- 
cess is capillarity, that power by which water rises or sinks 
in small passages according to whether it wets them or not, 
the energy being furnished by forces of tension within the 
liquid itself. Capillarity, however, plays but minor part 
in the physiology of most plants, though it has an indirect 
importance through its influence on the movements of water 
through soils. 

6. The Composition and Structure of Soils 

Roots have most intimate connections with soils, which 
must therefore be considered in connection with root physiol- 
ogy. Besides, soils have high interest on their own account, 
and because of their importance in agriculture. 

Soils are far more complex than they look, having no less 



238 A TEXTBOOK OF BOTANY [Ch. V, 6 

than six primary constituents, viz. pulverized rock, water, 
air, humus, dissolved substances, and micro-organisms. 
These are by no means intermingled without order, but have 
relations to one another which result incidentally in a 
kind of crude structure. 

Pulverized Rock. This constitutes the great bulk, 
fully 90 per cent, of ordinary soils. It is derived from the solid 
crust of the earth either by chemical decay of the rock or 
else by mechanical attrition. Attrition occurs by force of 
moving ice, as in glaciers (which have ground the surfaces of 
most northern countries), or else of running water, as in 
rivers, which forever are grinding the bowlders in their beds 
to fine silt. Thus we find every gradation, from great 
bowlders down through gravel and sand to silt and the finest 
clay. Under the microscope any soil presents the aspect 
of rough-angular fragments of rock, variously colored, and 
more or less crystalline. The weight and mutual pressure 
of these rock particles provide the resistance needful in the 
anchorage function of roots, while their irregularity in size 
and shape, forbidding a tight packing together, insures the 
open irregular spaces through which water and air can 
circulate in the soil. These features are well shown in our 
generalized drawing (Fig. 169). 

Water. This comes second in abundance though first in 
importance of the soil constituents. It furnishes the en- 
tire supply to ordinary plants, which can take none through 
their leaves or stems. It comes into the soil either direct 
from the rain or else by way of capillary movement up from 
lower levels. It is sometimes so plentiful as to saturate a soil, 
that is, fill its spaces completely, as occurs temporarily in all 
soils after drenching rains and permanently in bogs and 
swamps. Such a standing, or hydrostatic, condition of the 
water is not beneficial to ordinary plants, because, while 
supplying far more than they need, it displaces the air essen- 
tial to the respiration of the roots. As this too plentiful 
water drains or dries away, however, the larger spaces be- 



Ch. V, 6] 



STRUCTURE OF SOILS 



239 



come emptied, and refill with air, though the water still 
lingers in the smaller passages and angles in the capillary 
condition. Such a soil is moist, and its combination of 
water and air provides the very best conditions for roots, 
though one that is nowhere constantly found. It is the 
condition represented in our drawing (Fig. 169). As the 




Fig. 169. — A generalized drawing of a section, highly magnified, through 
a good soil and a portion of a root with root hairs. 

The soil particles are cross-lined, the water is concentrically-lined, the 
htimus is black, and the air spaces, in the soil, are left white. 



water is further removed, by evaporation and root absorp- 
tion, some moisture continues to cling tenaciously in thin 
films around the particles of soil, from which it is removed 
with greater and greater difficulty the thinner the films become. 
Upon these hygroscopic films plants must depend for their 
supply during much of the time ; and it is apparently for 
absorption from them that the root hairs, flattened tightly 
against the soil particles, are especially fitted (Fig. 170). 

The hygroscopic water films have an important relation 
with the soil particles. Not only do the films cling very 



240 



A TEXTBOOK OF BOTANY 



[Ch. V, 6 



closely to the particles, but they are themselves, through 
internal cohesion and surface tension, tenaciously strong; 
and thus they are brought into a state comparable with 
stretched rubber. On the other hand, the water molecules 
are extremely mobile within the films, as if they were the 
best ball bearings. From this combined tenacity and 
mobility of the films, it results that when water is with- 
drawn from any part of the soil, whether by root hairs or 
by evaporation, the films directly affected 
draw upon the others with which they are 
connected, and these upon others, so that 
the draft is thus made over a considerable 
distance. Hence a plant is not dependent 
for its water supply upon the soil with 
which its roots are in actual contact, but 
can draw from a far wider area. This ex- 
plains why a house plant dries out the soil 
of the pot uniformly ; how Cactus and other 
desert plants draw from great areas, growing 
well spaced apart ; and why deep homogene- 
ous soils, like those of the prairies, supply 

Fig. 170. — A ^ ' , ^ ^ \, > i^P J 

root hair in the Water SO evenly to crops, h urthermore, since 
soil, showing its ^j^g water films have in general the same 

intimate contact . , . i „ « . , . « . , ., 

with soil par- thickness regardless oi the size oi the sou 
tides; X 240 particles, a fine soil can retain more water 

(about). (After u- u • u i u u 

Strasburger.) than a coarse one, which is why clay holds 
more water than sand. 
Air. This forms the third in abundance of the constituents 
of ordinary soils, and is the source of the indispensible oxygen 
for the respiration of most roots. It fills the irregular spaces 
not occupied by water between the rock particles (Fig. 169) and 
is ordinarily continuous with the atmosphere above ground. 
In places of permanent hydrostatic water, like swamps, the 
air is excluded, and only such plants can there live as have 
large air passages to the roots from the leaves, or are able to 
absorb dissolved oxygen directly into their submerged bodies 




Ch. V, 6] STRUCTURE OF SOILS 241 

from the water. It is in order to introduce air into such 
soils that we drain them preparatory to growing crops. 

When air stands long in a soil, it loses part of its oxygen 
and accumulates carbon dioxide from root respiration. Ac- 
cordingly it is better for plants that this vitiated air should be 
expelled at intervals, and replaced by a fresh supply. Such 
a result accompanies soaking rains ; and the keeper of house 
plants does well to imitate the method by giving the plants 
an occasional thorough soaking, and allowing them to dry 
out in large part between times. Such treatment is much 
better than a frequent addition of small amounts, for the 
latter method does not effect renewal of air. 

Humus. This comes fourth in abundance of ordinary soil 
constituents. It comprises the dark-colored vegetable matter, 
mostly the remains of decaying roots, which to the eye of an 
expert is so characteristic a mark of a good' soil. A mixture 
of humus with sand and clay constitutes loam, the best of 
garden soils. The proportion of humus in soils varies greatly, 
from almost none through an optimum amount (represented 
in our picture (Fig. 169), to a very great deal, as in muck, 
which owes its black color thereto. Bogs consist almost 
wholly of a kind of humus, called peat, which only partially 
decays, and therefore accumulates. The value of humus in 
a soil, from the plant point of view, is four-fold. It lightens, 
or opens, a soil, thus increasing its aeration capacity ; it 
helps to retain moisture, being very absorbent; it adds 
substances, by its decay, to the soil solution, some beneficial 
and some harmful, though our knowledge of these matters is 
scanty as yet ; and most important of all, it supports 
numerous micro-organisms, which play a first role in soil 
fertihty. 

Dissolved Substances. In the soil water occur many 
dissolved substances, and therefore it becomes a soil solu- 
tion. Though profoundly important to plant life, the actual 
quantity of such substances present is relatively small, even 
the richest soil possessing only' a small fraction of 1 per cent al- 



242 A TEXTBOOK OF BOTANY [Ch. V, 6 

together. Most important are the mineral salts necessary in 
the nutrition of plants, and therefore commonly, though not 
quite correctly, called ' ' plant foods ' ' (page 28) . They consist 
in compounds of nitrogen, sulphur, phosphorus, magnesium, 
iron, potassium, and calcium, having the uses in the plant 
already described (page 230). They come into the soil so- 
lution chiefly through chemical disintegration of the rocks 
which contain them, but to some extent through action of 
living organisms, as will be further described a page or two 
later. These natural sources of supply are sufficient in 
case of wild plants, which, by decay, return their substance 
to the ground ; but under cultivation, \^ere great quantities 
of mineral matters are annually removed with the crops, 
some are apt to run short and must be replaced artificially, 
which is accomplished through fertilizers. The mineral 
salts which usually first become scarce are compounds con- 
taining nitrogen, phosphorus, and potash ; and since all 
three are abundant in barnyard manures, we can see the 
agricultural value thereof. Nitrates, phosphates, and potash 
salts, obtained from other sources, are also used commonly 
as fertilizers. Such, at least, is the older and, among 
farmers, still prevalent belief as to the role of fertilizers in 
the fertility of land. But of late some leading investigators 
have advocated a different view, based on the claim that the 
soil solution supplies all of the mineral salts which plants ordi- 
narily need, even on much-cropped land, the fertilizers 
finding their use chiefly in the neutralization of other un- 
favorable conditions in the soil. 

The functional use of the different mineral salts to plants 
is inferred from various lines of evidence, but chiefly from 
the results of water culture (Fig. 171). Many herbaceous 
plants can be grown from seed to maturity with the roots in 
water, their well-developed aeration systems providing suffi- 
cient oxygen to their roots. By using pure (distilled) water 
as a basis, it is possible to supply to a plant all of the neces- 
sary mineral salts except some given one, in which case the 



Ch. V, 6] 



STRUCTURE OF SOILS 



243 



t 



peculiarities of the resultant plant give a clew to the role of 
that substance. 

In addition to the mineral matters the soil solution con- 
tains small amounts of diverse organic substances, partly 
beneficial to plants and 
partly injurious. They 
are mostly set free by the 
decay of humus, which 
was originally living 
tissue containing pro- 
teins, carbohydrates, and 
other classes of sub- 
stances ; but some appear 
to be formed as excretions 
of living roots. It was 
an old belief, long aban- 
doned but now revived 
with new evidence, that 
roots excrete substances 
injurious to themselves, 
though commonly harm- 
less to other kinds ; that 
the accumulation of such 
substances tfends to poison 
a soil for the plants which 
produce them ; and that 
soils rendered barren by 
long use of one crop are 




Fig. 171. — Typical illustration of the 
methods and results of water culture ; 



The plants are Buckwheat. To dis- 
tilled water in the middle jar were added 
not exhausted of neces- all of the mineral salts needed by the plant; 

to that on the left, all except potassium ; 
to that on the right, all except iron. In 
the latter case the upper, less shaded, 
leaves are white, not green, in the plant. 
(Originally from works of Pfeffer.) 



sary mineral salts, as 
commonly supposed, but 
are poisoned by the ac- 
cumulation of these excre- 
tions. But these matters are still in debate, and their deci- 
sion must await further evidence. 

Micro-organisms. Last in prominence, though not in 



244 



A TEXTBOOK OF BOTANY 



[Ch. V, 6 



importance, of the soil constituents are certain minute liv- 
ing organisms, viz. Fungi, Bacteria, and Protozoa. 

Fungi, of certain small kinds, develop in contact with the 
tips of the roots of many plants, particularly such as Uve 
in much humus, weaving around them a close cover of my- 
celial threads, which replace the root hairs (Fig. 172). This 

MYCORHizA, as it is named, ab- 
sorbs water and mineral matters 
which it transmits to the roots; 
and there is some reason to be- 
lieve that it also absorbs solu- 
ble organic matters set free in 
decay of the humus but useful 
again to the plants. The associa- 
tion seems clearly beneficial both 
to fungus and flowering plant; 
and accordingly we have here 
one of the cases where two dif- 
ferent organisms derive benefit 
from their association, a condi- 
tion called SYMBIOSIS. Some 
kinds of soil Fungi seem also to 
have the same powers as Bacteria, 
next described, in relation to soil 
nitrogen. 

Bacteria, already known to the 
student as the smallest and 
simplest of living organisms, are abundant and of many kinds 
in all soils ; but the most important are those which effect 
NITRIFICATION and NITROGEN FIXATION. Nitrogeu, a con- 
stituent of the protoplasm, is one of the substances most 
indispensable to plants; but although it composes four 
fifths of the atmosphere, the higher plants are unable to take 
it from that source, and have to rely upon compounds ab- 
sorbed in solution through the roots. The presence of 
mineral salts containing combined nitrogen is therefore one 




Fig. 172.— Typical Mycorhiza, 
on the root of European Beech ; 
X 120. 

The entire root tip, back to 
beyond the hair zone, is com- 
pletely and closely covered by a 
felted mass of mycelial threads, 
which extend also into the soil. 
(After Frank and Tschirch.) 



Ch. V, 6] 



STRUCTURE OP SOILS 



245 



of the most important, perhaps the most important, factor 
underlying soil fertility. Moreover, the supply needs con- 
stant renewal to compensate for loss by drainage and removal 
from the land with the crops. Now it happens that some 
kinds of soil Bacteria have the power to change certain nitrog- 
enous substances, nota- 
bly ammonia, common 
in soils but not usable by 
the higher plants, into 
other nitrogenous sub- 
stances, notably nitrates, 
readily usable by those 
plants ; and such nitri- 
fication of soils, while it 
only transforms, and does 
not add nitrogen com- 
pounds, is yet an impor- 
tant element in soil fer- 
tility. Further, there are 
other kinds of soil Bac- 
teria which possess the 
power to take free nitro- 
gen from the air and 
incorporate it into com- 
pounds in their own 
bodies ; and such nitrogen 
fixation, on decay of their 
bodies, adds nitrogen to 
the soil, and is the chief 
source of supply in soils of 1?hat indispensable substance. 
Both kinds of Bacteria live in the humus, or at least are de- 
pendent thereon for most of their food, in which fact lies the 
principal reason for the association of humus with good soils. 
The nitrogen compounds formed by these Bacteria become 
ultimately dissolved in the soil solution, whence they are 
absorbed by the roots of higher plants. In a few families, 




Fig. 173. — Typical root nodules (or tu- 
bercles), on roots of Lupine ; X \. (Drawn 
from a photograph.) 



246 A TEXTBOOK OF BOTANY [Ch. V, 6 

however, and conspicuously the Pulse family, the relation is 
more direct, for the nitrogen-fixing Bacteria live . in the 
tissues, in the nodules so familiar on the roots of Beans 
and Peas (Fig. 173), to which the compounds are thus sup- 
plied with minimal loss. There is obvious connection be- 
tween this economical arrangement and the fact that the 
seeds of Leguminosse are richest of all plant products in 
nitrogenous substances, particularly proteins, thus coming 
nearest to meat in food value. 

The importance of nitrogen-fixing Bacteria in soil fertihty 
has of course suggested the attempt to enrich poor soils by 
adding the suitable Bacteria thereto. Many attempts have 
been made to this end, but while successful as laboratory 
experiments, they have not as yet achieved importance in 
practice. 

To complete the subject of nitrogen acquisition by the 
higher plants, we should note that such has been held to 
explain the insectivorous habits of the pitcher plants and 
others which trap insects (page 76). The plants which 
capture insects digest the bodies thereof, and absorb into 
their own tissues the resultant substances, which of course 
are particularly rich in nitrogenous materials. In general, 
the insectivorous plants are found in places where the 
nitrifying Bacteria of soils are unlikely to be found, — our 
Sarracenias and Sundews in bogs, the Venus Fly-trap in 
sand, and the Nepenthes on the trunks of trees. 

Protozoa are minute one-celled animals, typified by the 
creeping Amoeba. They abound in rich soils, the fertility 
of which they are now claimed to influence. It is found that 
any methods of treatment, by heat or poisons, which kill 
these Protozoa but not the Bacteria, produce increased 
fertility; and since it is likely the Protozoa feed upon 
Bacteria, the inference is drawn that the destruction of the 
former permits increase in numbers of the latter, with pro- 
portionally better nitrification and nitrogen-fixation. Here 
again, however, we must await further evidence. 



Ch. V, 7] SELF-ADJUSTMENTS OF ROOTS 247 

7. The Self-adjustments of Roots to Prevailing 
Conditions 

Roots possess in remarkable degree that property of in- 
dividual adjustment to the pecuHarities of their immediate 
surroundings, such as was earher described in the photo- 
tropism of leaves and the geotropism of stems. 

Geotropism, indeed, is no less characteristic of roots than 
of stems. The first root which issues from the germinating 
seed always grows over to point directly downward, no 
matter in what position the seed happens to lie (Fig. 119). 
It is described as positively geotropic, or progeotropic, 
the main stem being negatively geotropic, or apogeotropic. 
The secondary or side roots possess transverse geotropism, 
growing out horizontally, or nearly so, and are described [as 
DiAGEOTROPic. The tertiary roots, however, those which 
grow from the side roots, are hardly geotropic at all, and 
therefore respond more freely to the other influences next to 
be mentioned. The adaptive explanation of such geotropic 
growth is obvious, for thus the main root is brought in the 
quickest way to the water supply, essential to the further 
growth of the young plant ; the side roots are spread at angles 
which take them into the widest area of soil, while giving 
them angles advantageous to their anchorage function; 
and the tertiary roots are left free to wander wheresoever 
the materials needed by the plant are most abundant. 

Especially characteristic of roots is their hydrotropism, 
or sensitive adjustment to moisture in the soil. Roots 
not only grow towards soil moisture, but branch and grow 
more profusely in moist than in dry places. A practical 
exemplification thereof is found in the fiUing of drain pipes 
by tree roots (Fig. 174). The adaptive explanation of 
hydrotropism is sufficiently clear; since the primary func- 
tion of roots is the absorption of water, they need to find the 
most abundant supply. The actual operation seems to 
be this, — the tip of ^he VQOt is sensitive to differences in 



248 



A TEXTBOOK OF BOTANY 



[Ch. V, 7 



the quantities of moisture coming from different directions ; 
it transmits a suitable influence to the growth zone; this 
zone swings the tip over towards the moister side until the 

stimulus is even all around; 
then the root continues its 
growth in that direction. It 
is important to note that 
leaves and stems, neither of 
which absorb any water, are 
not in the least hydrotropic. 

A third self -adjustment of 
roots takes them towards air, 
— AEROTROPisM. Other things 
being equal, roots grow 
towards the places in the soil 
where air is most plentiful. 
At first sight it would seem 
that hydrotropism and aero- 
tropism must neutralize one 
another, since in general much 
water in the soil means little 
air, and vice versa. In fact, 
however, a complete satura- 
tion of the soil gives more 
water than plants can make 
use of, just as we have found 
that full summer sunlight gives 
more light than can be used 
by leaves (page 56) ; and it 
is towards the optimum com- 
bination of water and air, best 

Fig. 174. — Masses of roots, of a for root life, that rOOts are 

""xt^dttJirthlfirrLt^. g^^ided by their aerotropism, 

stick, cross-marked in decimeters, when their hydrotropism is 
The apparent cross folds mark the ^a+i^fip^ T| i^ bpcflUSP this 
collars where the tiles were joined. satlsnCQ. il IS Oecause tms 

(Drawn from a photograph.) optimum combination of watoi 




Ch. V, 7] SELF-ADJUSTMENTS OF ROOTS 249 

and air, with mineral matters, occurs commonly in drain 
pipes that roots are prone to enter and fill them. The 
adaptive ,explanation of aerotropism is of course very plain ; 
it is found in the need that all roots have for air (i.e. oxygen), 
indispensable to their respiration, which underlies all of 
their growth and work. 

Aerotropism is really but one phase of chemotropism, or 
self-adjustment to particular chemical substances, of which 
several forms are known. Thus, some roots grow towards 
a greater supply of the mineral substances they specially 
absorb, though their behavior in this respect is not always 
consistent, nor is it well understood. They show also several 
minor 'Hropisms, " of which traumatropism, or a turning 
away from injurious contacts or substances, is best known. 

All of these tropisms are typical cases of irritability, the 
equivalent of reflex action in animals (pages 55, 176). The 
response is not forced by the moisture, air, etc., but simply 
guided thereby, the work of turning being done by the plant. 
These phenomena in roots are especially interesting because 
the place of perception of the guiding stimulus is usually 
the growing point, while the place of response is the growth 
zone just behind it. Thus we have an arrangement com- 
parable with that in animals, where special sense organs 
receive the stimuli, and a separate muscular system makes 
the responses. 

In most plants the young main root is so strongly geotropic 
that it can be deflected only a limited amount from the 
vertical position by other influences, but the tertiary and 
later formed roots have so weak a geotropism that they 
respond to other stimuh very freely. And here must 
naturally arise this question : What happens in cases where 
two or more different stimuli act simultaneously upon the 
same root from different sides? In some few cases the 
stimuli seem to influence one another's action, but in general 
the root attempts to respond to them all. The position that 
is actually taken is then a resultant, depending upon the 



250 A TEXTBOOK OF BOTANY [Ch. V, 8 

directions of the stimuli and the relative sensitiveness of the 
roots thereto. 

The student will be interested to read at this point the 
fine passage which closes Darwin's book '^The Power of 
Movement in Plants." He should keep in mind, however, 
the fact that a thread of simile and fancy runs through the 
paragraph, to the matters of which our modern science now 
gives a somewhat more mechanistic interpretation. 

8. The Additional, and Substitute, Functions of 
Roots 

While the great majority of roots have the typical forms 
and functions already described, there are some which 
perform additional and even substitute functions, with 
corresponding modifications of structure. 

As in case of other plant organs, roots which perform the 
typical functions yet exhibit marked diversity of form, 
usually in clear correspondence with different habits. Thus 
the difference between the tap root and a mass of fibrous 
roots (page 212) is of this nature. Again, the relative im- 
portance of the absorbing and anchorage function varies 
much, the latter being highly important in great trees, and 
almost negligible in low herbs, especially such as have under- 
ground rootstocks ; and corresponding differences in struc- 
ture are manifest. The only roots of the low-growing Bryo- 
phytes are the great root hairs, or rhizoids, effective in 
absorption, but obviously having little utility, as there is 
Httle need, for anchorage. The depths to which typical 
roots descend vary also, for while those of swamp and bog 
plants keep near the surface, obviously in relation to air 
supply, those of some desert plants reach at least to sixty feet, 
as in the common desert shrub called Mesquite, evidently 
in adjustment to the water supply. And other differences 
are revealed by intensive study, — some most reasonably 
explained as adaptive, others as hereditary, and others as 
structural or incidental. Yet the diversity presented by 



Ch. V, 8] SPECIAL FUNCTIONS OF ROOTS 



251 



such roots is insignificant in comparison with the corre- 
sponding diversity in leaves and stems. The explanation 
is found no doubt in the fact that the conditions of life 
underground are much more uniform than conditions of life 
in the air. 

The commonest additional function of roots is storage, 
mostly of food, but partly of water, which occurs in all 
degrees, from so little as not to affect perceptibly the root 
shape, up to the pro- 
duction of a rotundly- 
swollen organ. The 
storage is oftenest in 
tap roots (which per- 
haps originated in this 
way), as familiar in 
Carrots, Beets, and 
Turnips. In other 
cases side roots are 
specialized, as exempli- 
fied in Dahlia and Sweet 
Potato, where they be- 
come tuberous (Fig. 
175), and would be 
hard to distinguish 
from stem tubers, were 
it not for the absence 

of eyes. Anatomically, as functionally, stem and root 
tubers are closely alike. As with stems, when starch is 
principally the food the texture is hard and white (e.g. 
Turnip) : when sugar, it is softer and translucent (e.g. Beet) : 
and when water, as in some desert plants, it is markedly 
succulent and almost transparent. 

Those flowering plants which have returned in their evolu- 
tion to a hfe in the water (e.g. Water Lihes and Pondweeds) 
exhibit naturally a marked reduction in the root system. 
Their roots are smaller in size and usually lack both hairs 




Fig. 175. 
Dahlia : 



— Typical tuberous roots, of 
X h (From Strasburger.) 



252 



A TEXTBOOK OF BOTANY 



[Ch. V, 8 




and the cap, though in some the cap is imitated by a pocket- 
like cover, and the hairs by slender side roots. The reduc- 
tion leads even to complete disappearance of roots in some 
immersed Pondweeds, which absorb through their leaf and 
stem surface, precisely as did their far-distant ancestors, 
the Algae. Herein we have a clear case of the widespread 

tendency for parts 
rendered useless by a 
change of habit to 
disappear gradually, 
often after lingering 
long in a rudimentary 
condition. 

There is in some 
plants another and 
very different correla- 
tion between root 
structure and water 
habit. Some com- 
mon herbs when 
grown in wet places 
develop at the contact of air and water a loose open tissue 
involving large intercellular spaces (aerenchyma), which 
seem to transmit air to the under- water parts. In a half- 
floating water plant called Jussicea repens (Fig. 176), some 
of the roots develop this tissue immensely, becoming enlarged 
to conspicuous white spindles which rise vertically to the 
surface, whence they evidently take air for use of the under- 
water parts. Thus we have roots with a new substitute 
function, that of aeration. The Bald Cypress, a prominent 
tree of the southern swamps, develops from its roots remark- 
able projections, or ''knees," which commonly reach the water 
surface, and are so constructed as to suggest theif use as aerat- 
ing organs for the roots, though such function has been denied. 
Other structures of analogous sort are found in Mangroves 
and elsewhere, as described in works upon water plants. 



Fig. 176. — Jussicea repens; X \. The swol- 
len structures are roots, composed chiefly of 
aerenchyma ; when young their tips reach the 
surface and they become filled with air, which 
later they supply to the parts under water. 
(After Goebel.) 



Ch. V, 8] SPECIAL FUNCTIONS OF ROOTS 



253 



In a few kinds of plants the roots are aerial, that is, fitted 
to Hve in the air, temporarily or permanently; and they 
exhibit corresponding modifications in structure. The 
simplest case is illustrated by Corn, where roots grow out on 
the stem a httle above ground and run diagonally to the 
soil, there acting as props to the stem, as well as organs of 
absorption. This arrangement is much farther developed 
in some tropical plants, notably the Screw Pine, or Pandanus 
(Fig. 177), where these roots come to form almost the whole 
support of the plant, the main stem remaining small. Similar 
roots, but more irregularly placed, 
cause the Mangroves of tropical 
shores to form their dense thickets. 
There are tropical plants, belonging 
mostly to the Fig family, of which 
the seeds germinate high up in the 
crotches of trees to which they 
happen to be carried; thence the 
growing plants send down aerial 
roots which, on reaching the ground, 
thicken to trunks so robust that 
often they strangle the supporting 
tree, leaving the strangler a several- 
trunked tree in its place. Analogous 
effects occasionally are seen in our 
own woods where trees have started on top of moss-covered 
bowlders, excepting that here the bowlder does not vanish. 
By similar aerial roots, put down vertically from horizontal 
branches and later developed to trunks, a single Banyan 
tree is enabled to spread to a many-trunked grove covering 
several acres (Fig. 178). There is no difficulty, of course, 
in understanding how roots can form trunks, since every- 
where behind their young tips all roots are practically stems 
in their structure (page 220). 

The extreme of the aerial habit is attained in some roots 
which never reach the ground. Thus, in the epiphytic 




Fig. 177. — Pandanus, 
showing the stilt-like roots. 
(From Balfour.) 



254 



A TEXTBOOK OF BOTANY 



[Ch. V, 8 



Orchids (Fig. 126), familiar in all greenhouses, while some of 
the roots penetrate the crevices of decaying bark and there- 
from absorb both water and mineral matters, others hang 
down free in the air. The latter display a distinctive, 
swollen, whitish aspect, the thickness being due to the pres- 
ence of many epidermal layers of loose, empty cells. Into 
these the water from rain is easily absorbed and thence 



r-xi^-^^ 




Fig. 178. — The Banyan, Ficus religiosa, of India. 
All degrees of development of the descending aerial roots appear in the 
view. The tree is a small and very open one. (From Balfour.) 



transferred to the fibro-vascular system, though the popular 
belief that such roots can absorb water as vapor from the air 
seems unfounded. In some kinds of Orchids the aerial roots, 
hanging in the light, show traces of chlorophyll, while 
in a few tropical kinds (Fig. 179) the roots become fully 
green, flatten to almost leaf-like thinness, and completely 
assume the photosynthetic function in place of the leaves, 
which are reduced to mere rudiments. Herein we have 
indeed a remarkable case of a complete substitute function 
in roots, and the one we would least expect, — that oi foliage. 
Aerial roots, of other forms, act as supports to climbing 
plants. Thus the true Ivies {e.g. the Enghsh Ivy and, of 
course, the Boston Ivy) put forth from the shaded sides of 



Ch. V, 8] SPECIAL FUNCTIONS OF ROOTS 



255 



their stems large numbers of short roots, tough in texture 
and adapted to drjniess, which adhere at their tips to stones 
or other supports (Fig. 180). In some tropical climbers, e.g. 
species of Ficus (Fig. 181), elongated aerial roots grow out 



t 



¥ 




Fig. 179. — ToBniophyllum Zollingeri, an epiphytic Orchid in which the 
aerial roots have become flattened and have assumed the function of foliage. 
A young flower stalk shows towards the left. (From Goebel.) 

horizontally, somewhat like tendrils, until they touch a 
support, around which they turn thigmotropically, thus 
securing a firm grasp; and it is interesting to note that 
such roots are able to swing in the horizontal plane by virtue 
of a marked lateral geotropism. Aerial roots also occur 



256 



A TEXTBOOK OF BOTANY 



[Ch. V, 8 



upon the trunks of Tree Ferns, which they sometimes cover 
completely with their stiff brown threads, though the. signifi- 
cance of their presence is not clear. 

A very striking modification of roots is found in the absorb- 
ing roots, or haustoria, of parasites. The common Dodder, 
for example, a parasitic twiner (Fig. 59), puts out these 
roots wherever it touches the host plant, and they, by use of 

digestive enzymes, penetrate the 
host, and attach themselves to 
the fibro-vascular bundles, from 
which they absorb nourishment. 
The haustorial function is simi- 
lar in other parasites, including 
the parasitic Fungi, though here 
the absorbing structures are my- 
celial threads, not true roots. 

Roots are transformed to spines 
in some Palms, though the signifi- 
cance of these structures is here 
no plainer than in case of leaves 
and stems (pages 79, 192). Also, 
some other, though minor, special 
functional uses and structural 
modifications of roots have been 
described. 

Though not matters of function, certain other special 
matters about roots may conveniently be noted at this 
place. Thus roots, like some stems, show anomalies of 
structure often very puzzhng, as, for example, the appear- 
ance in Beets which simulates annual rings. These rings are 
due to the fact that the cambium, after forming a certain 
amount of phloem and xylem, ceases to grow ; thereupon a 
new cambium arises just outside the first cylinder, grows for 
a time, is itself replaced, and so on, many times. Again, roots 
can shorten in length, especially in some perennial herbs, 
which thus are kept below ground despite the yearly growth 




Fig. 180. — English Ivy, 
showing the aerial roots by 
which it clings to walls. (From 
Le Maout and Decaisne.) 



Ch. V, 9] 



ECONOMICS OF ROOTS 



257 



of a ''crown" at the top. The shortening is effected by a 
forcible expansion of the shorter cross axes, and hence a con- 
traction of the longer vertical axes, of the cortical cells. An 
incidental result thereof is 
the formation, very marked 
in some tap roots, of cross 
wrinkles or folds, which 
are thus explained. 

Roots, like leaves and 
stems, exhibit abnormali- 
ties and monstrosities, 
though in less number and 
diversity than other parts, 
again no doubt because of 
the comparatively uniform 
conditions of life under- 
ground. On the other 
hand roots are especially 
subject to an influence 
from which leaves and 
stems are comparatively 
free, viz. the forcible im- 
position upon them of 
flattened, contorted, or 
other peculiar shapes, of- 
ten amusingly or gro- 
tesquely imitative of fa- 
miliar objects, by the rocks 
and other impediments 

among which they grow. Such growths are often unearthed 
and displayed as curiosities, or used in ''rustic" carpentry. 



HB 


^U^S 


^^^^^^'Hs 


1 -^P^^^^^^jIkm^ 


mWm 


^^^^^ 


JSm 


^^^^n 


MjjIllH 


^P^^P^m| 


^M 


^^^p 


R^^K^^ 


Im^^^f^^ 


i^^^^si 


^^^^m^i^^^KI 


w"'*^^BI 


^^^^^^o^fw§ 


^^^^ 


^^^^»fi^ 


^^5 


'^fMm^m 


^^^M 


m^^^K^^ 


I^^HI 


^^^^^^^1 


|T^^|MmM|^B| 


l^pl^^^^^Pfc^ r j^^i^~^RW 


Si ^'i^MW'SMiillMlw 


fflS^^^^^^^^^^K.^"'^ ^^^S3^i^ 


|«iy|H|||M 


^^^^^^^^^/^^^^ 


^H 


^^^H 


mM 


^^^^ 



Fig. 181 — A Ficus climbing by aid of 
aerial roots. (From Kerner.) 



9. The Economics, and Treatment in Cultivation, of 

Roots 

The most important economic uses of roots depend on 
the stores of food they contain, especially in the case of 



258 A TEXTBOOK OF BOTANY [Ch. V, 9 

biennials and herbaceous perennials, which store their food 
perforce underground. Beets, Carrots, Turnips, Radishes, 
and Sweet Potatoes are the most famihar examples. Not 
all, however, of the farmer's ^'root crops" are roots, for 
some are stems, as in potatoes, though this purely morpho- 
logical distinction has no importance whatever in economics, 
and very little in physiology. 

Because root-absorption is osmotic, and therefore requires 
a soil solution much weaker than the sap of the root hairs, 
it injures plants to over-fertilize them ; for fertilizers are solu- 
ble, and thus increase the strength of the soil solution. But 
the matter is also complicated by chemical relations, and 
the stimulation given to growth of soil Fungi and other 
organisms. 

Because roots need air for their respiration, wet or 
clayey soils must be drained for our crops, often at great 
trouble and expense. Yet water also is necessary, and must 
be conserved for dry times. The art of drainage consists 
in the maintenance of a beneficial balance between water 
and air throughout the growing season. It is because roots 
find this balance so excellent in drains, which therefore they 
tend to fill, that gardeners must take care where they place 
plants having specially hydrotropic roots (Willows, Poplars, 
Elms). Where a tree happens to grow on ground which 
must be graded to a higher level, a wall holding back the soil 
from the trunk will often permit enough aeration of the 
larger roots to save the tree, though frequently it does not. 

Like other parts, but perhaps more than they, roots need 
warmth when growing. This is why hot beds are used in 
the spring, — the heat being developed by the respiration of 
organisms producing fermentation or decay in the manures 
which are used. The same value inheres in '^bottom heat" 
supplied through pipes in the soil, sometimes used in forcing 
greenhouse plants. 

Any osmotic process, which root absorption is, proceeds at 
a rate directly proportional to temperature. Consequently 



Ch. V, 9] ECONOMICS OF ROOTS 259 

a low temperature permits only a slow water absorption, 
which explains the damage often done to trees and shrubs in 
early spring by warm weather and high winds while the 
ground is still cold, if not frozen ; for the resultant wither- 
ing, or windburn, and browning is caused by fatal drying 
through excessive transpiration unsupported by sufficient 
water supply (compare page 48). Much winter-kilhng of 
plants is due to the same cause, i.e. a similar exceptional 
loss of water through lenticels while the ground is still frozen. 
Thus it becomes a part of good gardening so to place and 
treat susceptible plants, by selection of sheltered situations 
or suitable coverings and the like, that they cannot be ex- 
posed to high transpiration conditions while the soil is chilled. 
The very different powers of plants to strike roots from 
cuttings or slips has important consequences in gardening. 
Some, e.g. the common house '^Geraniums," strike root 
very easily ; others, e.g. most ornamental shrubs, do so with 
some difficulty, and only when aided by special treatments ; 
while still others, e.g. our common fruit trees, will not do 
so at all. The causes of the differences are not fully known, 
but in general plants of succulent texture, with soft fibro- 
vascular system and plenty of stored food, strike root most 
easily. The matter is practically important in two ways, 
— first, plants which strike root from cuttings can thus 
be propagated rapidly and cheaply, and second, special 
varieties or sports can thus be preserved and multiplied 
indefinitely. The usual treatment of cuttings conforms 
perfectly to root physiology. The skilled gardener cuts 
usually just below a node, because the roots start most 
readily from nodes ; he removes much of the foliage, be- 
cause too much transpiration would dry 'out the tissues 
before the new roots could replace the supply ; he puts the 
cuttings first in sand, because ample air is the first requisite 
for the vigorous growth of new roots, but when rooted, he 
transplants them to loam for a better water and mineral 
supply. 



260 A TEXTBOOK OF BOTANY [Ch. V, 9 

Nurserymen are accustomed to transplant their trees and 
shrubs two or three times before sale, in order to force root 
formation near the stem. Thus more young roots are pre- 
served when the plant is lifted for shipment, and it has better 
chance for recovery when again planted out. This is a reason 
why nursery-grown trees and shrubs usually survive transplan- 
tation so much better than those brought from the woods. 

Much farming practice, ancient and modern, finds explana- 
tion in root physiology, though there is not agreement on all 
details. The addition of fertilizers, formerly interpreted as 
a replacement of needful mineral salts removed with the 
crop (page 242), may perhaps represent another method of 
neutralizing unfavorable soil conditions introduced by root 
excretion and decay. The ancient practice of letting a soil 
lie fallow (or idle), for a period, may restore the fertihty 
either by giving time for the diffusion of more mineral salts 
from lower levels, or through the removal by drainage or 
oxidation of the injurious substances formed by the roots. 
The rotation of crops may derive its value either from the 
different demands made by different crops upon the mineral 
supply of the soil, or from the fact that the organic materials 
added by roots are usually not injurious to other kinds of 
plants. There is, however, no question as to the reason for 
the value of the old practice of green-manuring, that is, 
plowing Clover, and other crops of the Pulse family, into the 
ground. These plants, with nitrogen-fixing Bacteria in their 
own roots, are very rich in nitrogen compounds which are 
thus added to the soil. Plowing is primarily of benefit in 
loosening a soil both for penetration of roots and admission 
of air. Suh-soil plowing carries the air still deeper, while 
likewise raising more mineral matters to the upper layers. 
The value of fall plowing, leaving the soil exposed for the 
winter, may possibly he partly in destruction of the Protozoa 
which are supposed to destroy the nitrifying Bacteria (page 
246). Cultivating 2l soil, in the gardening sense, consists in 
roughening the surface; it breaks the homogeneity of the 



Ch. V, 10] SUMMARY OF PLANT TISSUES 26l 

soil, lessens the continuity of the water films (page 240), 
and hence checks the free movement of soil water to the 
surface and its loss there by evaporation. The method is 
important in dry farming, of which the first problem is the 
conservation of soil moisture. 

10. Summary of the Functions and Tissues of Plants 

Having considered separately the principal tissues of 
plants, it will now be well to summarize them together 
to show their connections and the systems they form. 

The functions of plants are performed by their proto- 
plasm, which is subdivided into cells. When these cells 
are specialized to a particular function in considerable num- 
bers, we call them collectively a tissue. Organs are enlarge- 
ments or extensions of the plant body of such shape and 
position that they bring the tissues devoted to a function 
into advantageous relation with some external condition 
crucially affecting that function, — as witness the leaf in 
its relations to fight. Thus organs do not so mtich perform 
functions as permit tissues to perform functions in advanta- 
geous relation to external conditions essential to their 
operation. 

While some functions are performed almost wholly by the 
protoplasm inside the cells, others involve great specializa- 
tion of the shape, thickness, and other features of the cell 
walls. 

With respect to the tissues involved, all functions fall 
into four classes, as follows : 

I. Functions performed in tissues requiring special posi- 
tions and constituting morphological systems. 

1. Protection against unfavorable external influences, 
notably dryness of the air and entrance of parasites. The 
function requires external position, continuity, and a water- 
proof and '^germ-proof" structure. Tissue, epidermis, 
replaced later by cork, covering stems and leaves, but merg- 
ing on young roots to the root hair layer. This physi- 



262 A TEXTBOOK OF BOTANY [Ch. V, 10 

ologically-determined tissue has become fixed as a distinct 
morphological tissue system, called the dermal . system 
(Fig. 182). 

2. Photosynthesis, or formation of food under action 
of light. This function requires a superficial position, but 
protection by the epidermis, and hence comes immediately 
beneath the latter, which is transparent. Tissue, chloren- 
CHYMA, which covers young stems, and extends out into flat 
projections, the leaves ; in older stems ceases to be green, 
merges into bark, develops cork cambium and layers of 
cork in replacement of the vanishing epidermis, and grows 
to allow for expansion of the stem ; in roots it merges over 
into the cortical layer. Its innermost layer forms a special 
STARCH-SHEATH in youug stems, or endodermis in young 
roots, having perhaps a perceptive function. This tissue 
has become fixed as a distinct morphological system called 
the CORTEX (Fig. 182) or cortical system. 

3. Absorption of water, mineral matters, and carbon 
dioxide. This function requires an external position and 
freedom from interference of the epidermis. Tissue, for 
water and mineral matters, an external hair-layer on the 
roots, continuous with the epidermis of stem and leaves, and 
a thick cortical layer, continuous with the chlorenchyma. 
The absorption of carbon dioxide by the cortex involves 
no special tissues, but depends on stomatal breaks in the 
epidermis, and intercellular air spaces, which are especially 
large in chlorenchyma. 

4. Transpiration, incidental to gas absorption, involves 
no tissues, but the need for its control explains the presence 
of the regulating guard cells of the epidermis. 

5. Conduction of water and food through the plant. It 
requires a position in contact with the water-absorbing 
and food-making cortex, and an elongated tubular con- 
struction. Tissues, XYLEM, comprising ducts and asso- 
ciated elements : phloem, comprising sieve tubes and as- 
sociated elements : in leaves the bundle sheath, — the 



Ch. V, 10] SUMMARY OF PLANT TISSUES 



263 







Fig. 182. — Vertical section through a 
generalized plant, to show the typical ar- 
rangement of the tissue systems. 

Outside is the dermal system (double 
lined), formed by the dermatogen region in 
the bud, and providing the protective epi- 
dermis on leaf and stem, and the hair layer 
on the young root. Next is the cortical 
system (crosses), formed by the periblem 
region of the bud, and providing the chloren- 
chyma of leaf and young stem, with cortex 
of bark and young root. Next is the central 
cylinder, or stele, formed by the pleroroe 
region of the bud, and providing the vascular 
bundles, with the pith (coarse dotted). 
The bundles include food-conducting phloem 
(cross Hned), and water-conducting xylem 
(diagonally lined), with cambium (fine dotted) 
between them. The growing points of roots 
and stems (also fine dotted) with the cam- 
bium which unites them constitute the 
growth system. 



264 A TEXTBOOK OF BOTANY [Ch. V, 10 

three forming vascular bundles. These bundles unite 
frequently with one another into a cylindrical fibro-yascular 
system, which, repeated in all plants, has become a dis- 
tinct morphological system called the central cylinder, or 
STELE. This cylinder runs continuously throughout stems 
and roots, and into the branches (Fig. 182), but not into the 
leaves, which receive only separate individual bundles there- 
from ; for leaves consist simply of flattened masses of cortex, 
with epidermis, into which extend individual bundles from 
the central cylinder. 

6. Growth, or development of new tissues, primary, 
secondary, and general. Primary growth requires terminal 
position, as in buds and growing tips of roots, while secondary 
growth requires a position among the tissues to be contin- 
uously formed, as with cambium, which builds xylem and 
phloem. Tissue, meristem, which is primary in buds and 
root tips, and secondary in cambium ; but meristem itself 
can arise anew in young tissues, as in case of cork cambium, 
the growth layer of some endogenous stems (page 128), and 
roots like the Beet (page 257). The growth of tissues is not, 
however, confined to meristem, but can take place in young 
tissues by a general cell division, e.g. in expanding bark, 
fruits, etc. Collectively the meristematic tissues form a 
GROWTH, or MERISTEMATIC SYSTEM, which, in the exogenous 
type of structure, is continuous throughout the plant (Fig. 
182). 

Epidermis, cortex, and central cylinder, comprising tissues 
so fundamentally important in the life of the typical higher 
plants, have been repeated so long in evolutionary history 
that they have become fixed as morphological systems which 
are now regularly laid down in distinct tissue layers by the 
primary meristem of buds and root tips. Thus the tissue 
layer from which the epidermis develops is called dermato- 
GEN, that of the cortex periblem, and that of the central 
cylinder plerome. The plerome comprises a cylinder of fun- 
damental or parenchymatous tissue wherein are embedded 



Ch. V, 10] SUMMARY OF PLANT TISSUES 265 

certain strands of procambium, which develop cell for cell 
into the vascular bundles, while the tissue inclosed by them 
forms the pith, that between them the medullary rays, 
and that outside them the pericycle. If the procambium 
cells all turn into xylem and phloem, we have a closed bundle 
of the endogenous type, but if some of it turns into cam- 
bium, we have open, continuously-growing bundles of the 
exogenous type. In such cases the cambium usually 
extends across the medullary rays and forms a continuous 
cylinder. 

II. Functions performed in tissues requiring special posi- 
tions but not forming morphological systems. 

7. Reproduction, which is asexual or sexual. If asexual, 
the reproductive bodies are usually separable portions of thal- 
lus or shoot. If sexual, the reproductive bodies, the mother 
cells, spores, and sexual cells (later to be described), originate 
in areas of primary meristem persistent on certain branches 
(flowers). It requires a superficial position for fertilization 
and dissemination, and hence the reproductive tissues are 
developed in the cortex. 

8. Support of the foliage against strains of weight and 
winds. It requires, upon well-known mechanical principles, 
a tough, fibrous construction in a tube-form as near the 
exterior as practicable ; but, to permit movement of air, water, 
and food in and out through stems and roots, it is neces- 
sarily discontinuous, and hence mostly disposed in strands. 
The needs for this function vary greatly with the habits 
of plants. Tissues, collenchyma, just beneath epidermis, 
sclerenchyma in cortex or pericycle, bast fibers in phloem, 
WOOD fibers in xylem, while special strengthening sclereids 
occur in some exposed leaves. Because of the frequent 
presence of fibers with the vascular bundles, especially in 
herbaceous plants, these bundles are commonly designated 
fibro-vascular, even if no fibers are present. Collectively 
these tissues are sometimes described as forming a mechan- 
ical SYSTEM, though it has no morphological identity. 



266 A TEXTBOOK OF BOTANY [Ch. V, 10 

III. Functions performed not in special tissues but in special 
regions of the tissue systems. 

9. Storage of food and water. No special position is 
requisite ; hence it occurs in any available regions of pith, 
cortex, medullary rays, and other parts, which often, by 
multiplication of the cells, produce swollen tubers, bulbs, etc. 

10. Secretion and excretion. No special position is 
requisite, but usually occurs in special cells, as the idio- 
BLASTS, or collections of cells forming glands, resin pas- 
sages, etc. The latex system includes tubes ramifying 
throughout the other tissues (page 134). 

11. Dissemination. Involves no special tissues, but 
modifications, with special outgrowths of hairs, wings, 
hygroscopic walls, etc., of superficial tissues, epidermis, 
and cortex, as described in a later section. 

IV. Functions performed in all living cells. 

12. Metabolism, or chemical changes (apart from photo- 
synthesis and respiration) involved in the life processes. 

13. Respiration, or release of energy by oxidation. Re- 
quires usually the access of free oxygen, which is effected 
by development of an intekcellular air system (aeration 
system) ramifying everywhere throughout the plant, and 
opening to the atmosphere through stomata or lenticels. 
There occurs in some cases a special development of tissues 
with large air passages, called aerenchyma. 

14. Self-adjustment to immediate surroundings. Re- 
quires perception of the external stimulus, transmission to 
a motor zone, and a motor response mechanism. For all 
of these phases special structural features have been de- 
scribed, usually special modifications of tissues, as in case 
of the starch sheath (page 130). 



\ 



CHAPTER VI 
THE MORPHOLOGY AND PHYSIOLOGY' OF FLOWERS 

L The Distinctive Featuees of Flowers 

So prominent and distinctive a part of the plant is the 
flower, that to most people the study of flowers and the 
study of Botany are practically identical. The error of this 
belief is sufficiently attested by the proportions of the chap- 
ters in this book. Functionally the flower is the plant's 
principal organ of reproduction, being especially devoted to 
effecting fertilization, the central feature of sexual reproduc- 
tion. It is in connection with this function that flowers 
have developed the beautiful colors, attractive fragrance, 
and striking forms to which they owe their aesthetic charm. 
There seems indeed no limit to the variety they present in 
these respects. 

In color, flowers taken collectively display by far the great- 
est variegation found anywhere in nature; and somewhere 
among them could probably be matched any hue of the 
chromatic scale. Yet some true flowers lack any special 
color, being green like the foliage, as with Grasses, Birches, 
and Pines, which plants indeed are not popularly known to 
have flowers at all. Whatever the bright colors, they have 
usually this feature in common, that they contrast markedly 
with the background, being oftenest white or yellow if seen 
against foliage, blue or red if raised above it, and white if the 
flowers open at night. Furthermore, showy flowers usually 
stand out well beyond the foliage, and more than that, so ad- 
just themselves as to face the brightest hght. Thus flowers 
seem especially fitted to attract»the eye, as indeed we shall 
presently find that they are. 

267 



268 A TEXTBOOK OF BOTANY [Ch. VI, 1 

Flowers are also distinguished by odors, which, however, 
do not occur in all, perhaps not in the majority, at least so 
far as our comparatively dull sense can perceive. When 
present, they are usually pleasing, or fragrant, to us, though 
a few, like Skunk Cabbage, are positively repulsive. Con- 
trary to a popular belief, fragrant odors and bright colors do 
not commonly go together, at least not in wild plants ; for 
in general the most fragrance occurs in flowers of which the 
colors are not especially visible, while odors are unusual in 
plants of exposed habit, — in meadows, roadsides, or prairies. 
Thus odors supplement a certain defect of color in making 
known the presence of flowers. 

In size, flowers range very widely. The typical size in 
those which occur separately would perhaps exceed somewhat 
an inch across ; but thence they range down to almost mi- 
croscopic dimensions, as in certain small floating water weeds, 
and upward to the truly gigantic proportions of the tropical 
Rafflesia, in which a single flower is more than three feet 
across (Fig. 61). Everybody knows that some connection 
exists between flowers and insects, and it is true in general, 
as we shall presently see, that a relation exists between flower 
size and insect size. Between flower size and conspicuous- 
ness, there also exists a correlation in this way, that while 
the larger flowers are commonly solitary, the smaller occur 
massed together into clusters, acquiring thus a collective 
prominence, the perfection of which is found in the com- 
posite heads of the Daisy, Sunflower, and others of their 
family. 

In shape, flowers are strikingly multiform. Simplest are 
those which have no showy parts at all, but only the incon- 
spicuous sexual parts. Of the more familiar kinds, some 
are regularly concave, like Buttercups and Apple blossoms, 
with the sexual parts in the center, but thence they range 
to such elongated forms as the Fuchsia, and, becoming ir- 
regular, produce bizarre effects in the Orchids, even to a 
degree simulative of insects or other unrelated natural ob- 



Ch. VI, 2] STRUCTURE OF FLOWERS 269 

jects. Between these shapes and real insects there exist 
certain relations presently to be noted. 

It is also characteristic of flowers to be fleeting, for com- 
monly they last but a few days, after which they wither and 
fall. Sometimes indeed they keep fresh but a few hours, and 
it is only very rarely that their substance is sufficiently firm 
to per-sist after drying, though this does occur in the kinds 
we call '^ Everlasting." Not the entire flower, however, 
perishes with the showy structures, for the central parts 
persist and grow gradually to the fruit which contains the 
seeds. Indeed, it is the most normal and characteristic 
feature of the flower that it precedes the fruit. The popular 
idea that the flower is in some way essential to the production 
of seed is thus correct. 

In case of the flower, as of o^her plant parts, the popular 
and the scientific conceptions are by no means coincident. 
The botanist includes under the term any structures, no mat- 
ter how minute and obscure, which have part in the pro- 
duction of seed, and excludes from the term any structures, 
no matter how flower-like, which have no such function. 
Thus, the so-called ''moss flowers" are not botanically 
flowers at all, and much lees are ''wooden flowers," "flowers 
of tan," and some other objects to which the name is fanci- 
fully applied. 

2. The Structure of Flowers 

Despite their striking external multiformity, flowers are 
comparatively simple and uniform in their mode of con- 
struction. 

A typical simple flower, such for example as the Peony 
(Fig. 183), has six or seven distinct kinds of parts. 

Outside is the calyx, composed of a whorl of five green and 
leaf -like sepals. In the unopened bud they form a close 
protecting cover to the parts inside them, wherein consists 
obviously their function. Usually they are somewhat 
triangular in shape, opening out in a star form, but often 



270 



A TEXTBOOK OF BOTANY 



[Ch. VI, 2 



they are rounded (as in the Peony), or else elongated, or 
otherwise shaped. Typically green like leaves, they some- 
times assume both the color and shapes of the next inner 
parts, the petals, as with Anemone and Four-o'clock. Usu- 
ally persisting for a time in the opened flower, they some- 
times fall off as it opens, as in Poppies. Commonly composed 
of separate sepals (polysepalous), the calyx is oftCTi one 
piece (gamosepalous), forming a saucer-, cup-, urn-, or tube- 
shaped structure, from the summit of which the free sepals 

project. Oftenest five 
in number, the sepals 
may be two, three, 
four, six, or more, in 
lessening frequency. 
The student may 
easily confirm all of 
these matters for him- 
self, and extend them, 
in any garden or green- 
house. 

Next inside the 
calyx comes the co- 
rolla, formed, in the 
Peony, of a whorl of 
five brightly-colored 
PETALS. Collectively they open out in a way to display a disk 
of color surrounding the sexual parts ; and herein, as will later 
appear, consists their function, — that of showing to insects 
the position of those parts. The separate petals are here 
broadest towards their tips, with narrow bases ; but from 
this typical condition there are wide deviations. The bases 
are extended into greatly elongated stalks, as in Carnation ; 
or their tips are pointed, elongated, cleft, fringed, and vari- 
ously formed, as the flowers of any greenhouse or garden il- 
lustrate ; while the most remarkable spurs, hoods, and other 
structures occur, as in Larkspur and Columbine (Fig. 207). 




Fig. 183. — A typical flower, of Pceonia 
peregrina ; X f . 
Some of the sepals and petals have been 
removed in order to show clearly the stamens, 
a, and the pistils, g. (From Strasburger.) 



Ch. VI, 2] 



STRUCTURE OF FLOWERS 



271 



at, 



In fact it would seem as though every shape that fancy can sug- 
gest must be embodied in the shapes of the corollas of flowers. 
In conformity with their fleeting character, they are delicate 
in texture, showing clearly through a lens or the microscope a 
leaf -like anatomy of veins and 
cortical cells, — the latter held 
tensely expanded by the os- 
motic pressure within. Typ- 
ically composed of separate 
petals (poLYPETALOUs), the 
corolla is often, like the calyx, 
one piece (gamopetalous) , 
forming a saucer-, cup-, urn-, 
or tube-shaped structure, 
from the summit of which 
spread the free petals, as 
Primrose (Fig. 201), and 
other garden flowers illus- 
trate. Usually standing di- 
rectly on the enlarged end 
of the flower stalk, or recep- 
tacle, the petals often stand 
on the calyx, as in Garden 
Nasturtium. As to number, 
petals vary like the sepals, 
being usually, though not al- 
ways, the same number as 
they. Not infrequently the 
petals are wanting altogether, 
in which case the sepals often 
replace them in color and 
function. Sometimes the sepals are likewise absent, and 
in this case the flower is very inconspicuous unless color 
is supplied by bracts beneath the flower (page 74), as in 
Poinsettia. As to the variety of colors displayed by corollas, 
we have spoken already. 




Fig. 184. — A typical simple ovule, 
of Polygonum divaricatum, in section ; 
X 135. 

e, egg cell, in the embryo sac, 
which lies in a mass of tissue, the 
nucellus ; fun is the funiculus or stalk 
by which the ovule is attached to the 
ovary ; ai and ii are the integu- 
ments, developed from the funiculus, 
and leaving an open micropyle ; cha, 
the chalaza. (From Strasburger.) 



272 



A TEXTBOOK OF BOTANY 



[Ch. VI, 2 




Next inside the corolla comes the whorl of stamens. 
Each consists of a slender cylindrical stalk, the filament, 
bearing at its tip an enlarged rounded yellow anther. A 
lens will show that this anther opens by longitudinal slits, 
allowing the escape of a yellow powder (each grain of which 
is a cell), called pollen. Pollen contains the male sex cells 
of the plant, which later fertilize the female sex cells in the 
ovules, as presently to be noted. The filaments are some- 
times short, even to disappearance, but again are elongated 
almost to thread-like, as in Night-blooming Cereus. Often- 

est cylindrical, they are 
sometimes flat and even 
petal-like, as in Water 
lilies, where it is difficult 
to say whether the fila- 
ments are petal-like or 
petals bear the anthers. 
The anthers, typically 
rounded, are sometimes 
greatly elongated, or 
forked, or otherwise re- 
markably shaped, while 
they open in very diverse ways. Usually forming a direct exten- 
sion of the filament, they are sometimes balanced on the point 
thereof, as familiar in the larger Lilies.. Commonly quite 
separate from one another (polyadelphous), they some- 
times form one piece at their bases (monadelphous), as 
in some of the Pulse family, and Abutilon. While typically 
they stand directly on the receptacle, they are often on 
corolla or calyx. Their existence is fleeting like that of the 
corolla, with which they commonly fall. While often 
numerous, as in Peony, Buttercup, and Rose, they are 
commonly limited in number, being typically the same num- 
ber, or double the number, of the petals, and therefore 
oftenest five or ten, or else less frequently three or six, or four 
or eight, though other numbers occur with lesser frequency. 



Fig. 185. — Generalized forms of 
ovules, showing the principal arrange- 
ments on the stalks ; magnified. 

Left, upright, orthotropous, form like 
Fig. 184 ; middle, inverted on elongated 
attached stalk (raphe), anatropous ; right, 
curved over to inversion, campylotropous . 
(Reduced from Strasburger.) 



Ch. VI, 2] 



STRUCTURE OF FLOWERS 



273 



When the same in number with the petals, the stamens 
usually alternate therewith,, though they stand opposite in 
Primrose (Fig. 201). Flowers also occur without stamens, 
or rather, to be exact, with the stamens and pistils in 
separate flowers. 

Beyond the stamens, and occupying the center of the 
flower, come the pistils, made up of parts called carpels, 
which, like sepals, petals, and stamens, are morphologically 
leaves. The Peony here pictured (Fig. 183) shows two 
pistils composed each of a carpel ; and of one carpel each are 
the many pistils of Buttercup and Strawberry, while Peas 
and Beans have one pistil composed of one carpel. Most 




Fig. 186. — Typical forms of nectaries. From the left ; spurs of Toad- 
flax, receptacular swellings of Grape, spurs of Columbine, scales on the 
petals of Ranunculus. (From Bailey.) 



commonly, however, there is one pistil composed of several 
united carpels. Whether simple (of single carpels), or 
COMPOUND (of several carpels united) , the pistil has typically 
a rounded hollow base called the ovary, tapering upward 
to a short cylindrical stalk called the style (very short in 
the Peony), ending in a roughened area called the stigma. 
When the ovary is opened, it is found to contain a number 
of small rounded whitish bodies called ovules, within 
each of which, in a special sac, lies a female sex cell, called 
the egg cell. The functions of the parts of the pistil are 
plain; the stigma receives the pollen containing the male 
cells, the style supports the stigma in a position suitable 
for receiving the pollen, and the ovary protects the dehcate 



274 



A TEXTBOOK OF BOTANY 



[Ch. VI, 2 



ovules in which the egg cells are fertilized by a method 
to be fully described in the following section. 

The ovules are the most important parts of the pistils. 
A typical ovule when mature shows the structure represented 
by Figure 184. Innermost is the relatively large embryo 
SAC (large enough to be seen by the naked eye in sections 
of very large ovules) , containing much protoplasm which can 
be recognized by its yellowish-brown color. In the sac lie 
also several small cells (Fig. 190), amongst which is the one 
of greatest importance, usually the 
largest, the egg cell. This egg cell 
when fertilized grows gradually to an 
embryo plant within the embryo sac. 
The embryo sac is imbedded within 
a rounded mass of tissue called the 
NUCELLUS, which in turn is inclosed 
by one or two integuments. These 
grow up to surround the nucellus from 
the stalk or funiculus, though they 
never inclose the nucellus completely ; 
for an opening called the micropyle 
is always left for the entrance of the 
pollen tube (Fig. 190). The funiculus 
is sometimes straight, but oftener is 
elongated and grown to the rest of the ovule, the resultant 
ridge being called the raphe. In this way the ovule is swung 
into positions facilitating the entrance of the pollen tube into 
the micropyle (Fig. 185). Through the funiculus runs a 
vein which conveys food into the ovule as far as the meeting 
place, called the chalaza, of nucellus, coats, and funiculus, 
whence it passes by diffusion to the various parts. 

Typically rounded in form, the ovary is often elongated, 
cylindrically as in Peas, flattened as in Beans, or variously 
angular. The style is almost wanting in the Peony, but in 
some flowers is elongated even to thread-like, as conspicuously 
in the silk of the Corn, while at times it is flat and quite petal- 




FiG. 187. — The Snap- 
dragon, a typical irregular 
(bilabiate) flower ; X f . 
(From Le Maout and 
Decaisne.) 



Ch. VI, 2] 



STRUCTURE OF FLOWERS 



275 




I 



like, as in Iris (Fig. 199). The stigma, while frequently flat, 
or rounded, is elongated variously, and even branched, some- 
times to almost a feather-like degree, as in some trees (Fig. 
197). The ovules, while typically few, are sometimes but one 
to a carpel, though often they are many, as with Poppies, in 
which case they stand in regular 
masses or rows upon supporting 
ridges, usually prominent, called 
PLACENTA. Unlike the other 
parts, the pistils are not fleeting 
but persistent structures, for, 
after fertilization, the pistils 
grow normally into fruits, and 
the ovules into seeds ; at least 
this is true of the ovaries, though 
commonly the styles and stigmas 
fall with the petals. While typ- 
ically the pistil or pistils stand 
on the receptacle separate from 
the other parts, frequently the 
other parts stand upon the ovary, 
bringing the latter below and 
outside of the showy part of 
the flower, as with Apple and 
Fuchsia ; and in this case the 
ovary is described as inferior, 
as contrasted with the ordinary 
SUPERIOR condition. 

Three other less prominent 
parts appertain to flowers : the receptacle, nectaries, and 
bracts. 

The RECEPTACLE is the tip of the floral stem, usually en- 
larged to a club-shape where it bears the floral parts, though 
sometimes it develops specialized forms, as will later be 
noted in the section on morphology. Nectaries excrete, 
often in shining drops, the nectar upon which insects feed. 



Fig. 188. — The structure of 
a pollen grain, and germination 
of the pollen tube, in Lilium 
Martagon; X 200. 

A, ripe grain with its own vege- 
tative nucleus, k, and a generative 
cell, m, which produces the two 
sperm nuclei ; B, the same grain, 
in another view, about to germi- 
nate ; C the germinated grain 
with its tube, the growth of which 
is controlled by the nucleus k ; 
D, end of the tube, with its two 
sperm nuclei, formed from the cell 
m, as it nears the egg cell. (From 
Strasburger.) 



276 



A TEXTBOOK OF BOTANY 



[Ch. VI, 3 



Typically they . occur at the bases of the petals, either as 
glandular swellings, or else as shallow cups, as seen, so con- 
spicuously in the Crown Imperial, where they hold huge 
ghstening drops ; and sometimes they are parts of the re- 
ceptacle (Fig. 186). Often they are wanting, in which case 
the nectar is usually secreted from the base of the petals 
without special glands. Bracts are leaves belonging to 
the stem below the flower, but often cooperating in the con- 
struction or function of the flower. Sometimes they consti- 
tute the conspicuous color-bearing part 
(as in Poiiisettia or BougainvillcBa) ; but 
again they are merely accessory sepal- 
like appendages, as in Strawberry, or 
else they replace the sepals in both 
form and function, as in Sunflower 
and others of the Composite family. 

Typically a flower is symmetrically 
radiate in form around a central axis, 
such kinds being called eegular, as 
with all of those we have thus far 
mentioned. But many are irregular, 
with a special tendency to form a two- 
hpped structure, as in the Mints and 
Snapdragon (Fig. 187), while this char- 
acter attains a very extreme development in the Orchids. 
Its significance will soon be explained. 

Various parts of the flower can be wanting to such a de- 
gree that pistils only or stamens only remain, thus forming 
PISTILLATE and STAMiNATE flowcrs respectively. All that is 
really essential to a flower is the possession of a stamen or a 
pistil. A flower which contains both is said to be perfect, and 
one which has also petals and sepals is said to be complete. 




Fig. , 189. — a pollen 
grain germinating on a 
stigma. (From Sachs.) 



3. The Accomplishment of Fertilization by Flowers 

Everybody knows that flowers are essential to the produc- 
tion of seed, though not everybody knows in what way. 



Ch. VI, 3] FERTILIZATION IN FLOWERS 



277 



Their function consists in effecting fertilization, the union 
of male and female sex cells, without which process seed 
does not form. The accomplishment of fertilization is the 
primary function of the flower. 

Fertilization in flowers involves three stages, two of them 
preliminary and subsidi- 
ary to the third and 
crucial one, — viz. pol- 
lination, growth of the 
pollen tube, and fusion 
of the sex cells. 

The pollen grains are 
developed in the anthers, 
and develop within them- 
selves the male, or sperm, 
cells (Fig. 188). The 
transport of the ripe 
pollen from anthers to 
stigmas, through a space 
sometimes small but fre- 
quently great, is called 
POLLINATION. It is not 
effected by any power 
within the plant, but by 
some external agency, — 
mostly by wind in the 
inconspicuous flowers, 
and by insects in con- 
spicuous ones. 

Pollination accomphshed, the growth of the pollen tube 
begins. Into the roughened, sugary-adhesive, epidermless 
surface of the stigma (Fig. 189) there grows from the pollen 
grain a slender, delicate, thin-walled tube, in which can be 
seen the distinctive living protoplasm. This tube, carry- 
ing the two sperm nuclei near its tip, grows down through 
the tissues of the style, dissolving for itself a way by ajd of 




Fig. 190. — A typical ovule, of Narcissus, 
ready for fertilization, in section; much 
magnified. 

Near the upper end of the embryo sac 
lie three cells, of which the larger is the egg 
cell. The pollen tube is shown entering 
the micropyle. (Drawn from a wall chart 
by Dodel-Port.) 



278 



A TEXTBOOK OF BOTANY 



[Ch. VI, 3 



enzymes and absorbing the digested products for use in its 
own further growth; and commonly the tube dies and 
withers behind as the forward part advances. Thus it 
reaches and enters the ovary, and, taking a direct path, 
approaches an ovule which it enters by the micropylar 

opening; thus it reaches the rela- 
tively large sac, the embryo sac, 
which every ovule contains (Fig. 
190), and within which, near the 
micropyle, lies the egg cell. This 
mechanism of fertiUzation is repre- 
sented in principle in our general- 
ized picture (Fig. 191). Thus are 
the sperm cells brought to the im- 
mediate vicinity of the egg cells. 

The final, and really the essen- 
tial, stage in this process consists 
in the fusion of the two sex cells 
which is thus effected (Fig. 192). 
One of the two male nuclei (the 
fate of the*other will appear later 
in connection with the fruit), with 
probably some surrounding cyto- 
plasm, moves out of the pollen 
tube into the egg cell, presumably 
opening the way by action of 
enzymes. For a time the egg cell 
exhibits two nuclei ; but they move 
together, touch, and then gradually 
fuse together into one and fertili- 
zation is complete. The result is a cell containing a nucleus 
derived from the union of two nuclei from different parent 
cells ; and this appears to be the central and essential feature 
of all fertilization. 

After fertilization the sepals, petals, stamens, nectaries, 
styles, and stigmas, their functions evidently accomplished, 




Fig. 191. — A generalized 
pistil and ovule, in section, 
showing the mechanism of fer- 
tilization described in the text. 



Ch. VI, 4] NATURE OF FERTILIZATION 



279 



i 



ordinarily wither and fall away, 
leaving only the ovary on the re- 
ceptacle. Then this ovary grows 
into a fruit, the ovule into a seed, 
and the fertilized egg cell into an 
embryo plant. In case, however, 
no fertilization is effected, the parts 
of the flower usually persist some- 
what longer than otherwise, though 
no fruit, seed, or embryo is 
formed; but presently all parts, 
including the ovary, wither and 
fall. This is the way in which 
flowers are essential to the pro- 
duction of seed. 

4. The Nature and Conse- 
quences OF Fertilization 

Fertihzation in flowers, as the 
preceding section has shown, cen- 
ters in the fusion of the male and fig. 192. — The fusion of 
female nuclei within the egg cell ; the sex cells, somewhat gen- 

- IT , • 1 ,1 ii c eralized from a typical case; 

for pollination and the growth or x 375. 

the pollen tube are merely the ^' the end of the pollen 

1 . „ 1 . . ^1 tube (Z) of Fig. 188), contain- 

mechamsm for bringing the sex ing two sperm nuclei, sk; B, 

cells together. Fertilization occurs the same tube in contact with 
., 1 /• /• 1 n an embryo sac, en ; C, a sperm 

m the reproduction of nearly all nucleus, sk, has entered the 
plants and animals, and while the egg cell, the nucleus of which, 

, . „ , . . , , ek, it has approached ; D, the 

mechanisms for bringing the sex sperm nucleus, sk, has lost its 

cells together are as diverse as elongated form and become 
., , ,, j^ 1 r« V c ,1 rounded like the egg nucleus, 

possible, the central feature of the ^th which next it fuses com- 

fusion, especially of the nuclei, is Pletely. (Reduced from Stras- 

always the same. Thus this fusion ^^^^^' 
act of fertilization runs as a thread of structural and physi- 
ological identity almost throughout the plant and animal 
kingdoms, binding plants and animals together in this 




280 A TEXTBOOK OF BOTANY [Ch. VI, 4 

one fundamental feature, despite their diversity in other 
respects. 

So certain, indeed, is the identity of the sex cells through- 
out animals and plants that of late some botanists have 
adopted from zoology the terms egg and spermatozoid, 
and use them instead of the older terms egg cell and sperm 
cell. The latter words are retained in this book, because while 
indicating sufficiently the morphological relations, they 
are more closely associated with the study of plants. Egg 
cells and sperm cells are called collectively germ cells. 

We examine first the cytological basis of the fusion of the 
germ cells. The student will recall . that the protoplasm 
in a hving cell is differentiated into the cytoplasm, which is 
clearly the constructing part, the plastids, which have 
chemical functions, and the nucleus which seems to act as 
a control center of the work and development of the cell 
(page 41, Fig. 16). Now as to the nucleus, its essential 
part is its chromatin, which lies embedded in the gelatinous 
protoplasm constituting most of its substance. This chro- 
matin is a distinct chemical substance, colorless in the 
living cell, but possessing a remarkable power to absorb 
colored stains (whence of course its name) ; and it ordinarily 
exists in fine granules strung together in threads which are 
much intertangled (Fig. 193, 1). This is its usual, or resting, 
state, but when the cells are about to divide, in reproduction 
or growth, the chromatin threads disentangle themselves 
and collect into definite elongated bodies called chromo- 
somes (Fig. 193, 7). The number thereof is always definite 
for each kind of plant or animal, though varying widely 
for different kinds, — all the way, in fact, from two to more 
than one hundred. Whatever the number, however, there 
is good reason to believe that they keep their identity even 
in the resting state, so that the chromosomes which come 
out of the seeming tangle are identically the same ones which 
went into it. Now in the fusion of the nuclei of the germ 
cells, the sperm nucleus passes over its chromosomes to the 



Ch. VI, 4] NATURE OF FERTILIZATION 



281 



egg nucleus, as shown diagrammatically in Fig. 194. It is 
true, the chromatin is in the resting state at this time, so 
that the chromosomes cannot be distinguished; but at the 




Fig. 193. — Stages in typical cell division in growing tissue, somewhat 
generalized. 

1, Resting cell. 2-5, The chromatin collecting into chromosomes, which 
become definite in number and outline. 6, Splitting of each chromosome 
lengthwise into two. 7, 8, The chromosomes growing shorter and thicker. 
9, 10, They collect in the equatorial plate of the forming spindle. 11, 12, 
13, Separation of the pairs of chromosomes. 14, 15, 16, Formation of the new 
nuclei, return of the chromosomes to the chromatin threads, and formation 
of a new wall. Thus are produced two new cells from division of the original 
one. Next these daughter cells grow to the full size of the parent cell, 
after which they again divide in the same manner. (After Strasburger.) 



282 



A TEXTBOOK OF BOTANY 



[Ch. VI, 4 



very first division of the fertilized egg cell its nucleus dis- 
plays a number of chromosomes precisely equal to that which 
we know the sperm and egg nuclei possess together, while 
in some cases it is found possible to identify the two sets. 
All evidence seems to indicate that this bringing together 
of the chromosomes (or chromatin) from two parent cells 
into a single nucleus is the principal (and perhaps only) 
functional end of fertilization. The accompanying com- 
minghng of the cytoplasm appears not to be important, 
and indeed in some cases seems not to occur, only the nucleus 
of the sperm cell passing into the egg cell. 

Thus we are brought to ask what may be the significance 





Fig. 194. — Diagram to illustrate the performance of the chromosomes 
in the fusion of the germ cells. 

The two chromosomes from the pollen or male parent are white, and those 
from the egg cell or female parent are black. 



of the chromosomes. Our knowledge of them is still im- 
perfect, but this much seems clear, — that they are the phys- 
ical basis of heredity, the carriers of the factors which cause 
the new individual to develop in general like its parents. 
The chromosomes do not themselves build the cells out of 
which the organism is constructed, for that is clearly done 
by the cytoplasm ; but in some way, still unknown, the 
constructive work of the cytoplasm seems guided by the 
chromosomes, which thus must contain the plans, so to 
speak, of the new structure. Furthermore, and this is im- 
portant, the evidence indicates that the set of chromosomes 
contributed by each nucleus in fertihzation is complete, that 
is, capable of guiding the construction of a complete organism 



Ch. VI, 4] NATURE OF FERTILIZATION 



283 



substantially like that which produced it. Thus the nucleus 
of the fertiUzed egg cell contains chromosomes in duplicate, i.e. 
two complete sets, each capable of reproducing an individual. 
The subject becomes clearer as we consider the events 
which follov/ fertilization. The fertilized egg cell at once pro- 
ceeds to divide. First the chromatin threads collect them- 
selves into definite chromosomes, equal in number to those 
contributed by the two nuclei, and indeed, there is little 
doubt, individually identical therewith. Then each chromo- 
some sphts lengthwise very regularly into two (Fig. 193), 
whereupon, by action of a spindle-shaped mechanism, 




Fig. 195. — Diagram to illustrate the performance of the chromosomes 
in the division of the fertilized egg cell. 

On the left the same egg cell shown in Fig. 194, its chromosomes being 
derived from the two parents ; on the right the two cells resulting from 
its division. 

one of the half chromosomes in every case is drawn to one 
end of the cell, and the other half to the other, thus dividing 
every one of the original chromosomes equally between the 
two ends of the cell. Then a wall forms across the cell be- 
tween them, and the two sets merge back each to its chroma- 
tin tangle. Thereafter these two half-sized cells absorb 
nourishment and gradually grow to the full original size, the 
chromatin included. Presently these new cells themselves 
divide, and the chromosomes which differentiate from the 
chromatin tangles seem identical with those which went 
into them, and therefore are the grown-up halves of the 



284 



A TEXTBOOK OF BOTANY 



[Ch. VI, 4 



original male and female set (Fig. 195). Then follow 
other like divisions, always by the same method, . though 
presently proceeding more actively in some places than 
others, — with the result that first an embryo, then a seed- 
ling, and finally an 
adult plant is pro- 
duced. When, now, 
the cells of the adult 
are examined, their 
nuclei are found 
each to contain ex- 
actly the same num- 
ber of chromosomes 
as did the fertilized 
egg cell from which 
the plant has de- 
veloped; and fur- 
thermore it seems 
reasonably certain 
that these chromo- 
somes of the adult 
are the exact indi- 
vidual descendants 
of those in the fer- 
tihzed egg cell, and 
therefore of those 
contributed by the 
original parent male 
and female cells. 
This phase of our 
subject appears clearly in a later diagram (Fig. 219). Thus 
the adult would have in every cell of its structure chromatin 
matter, that is, heredity material, derived from both of its 
parents. This arrangement helps us to understand how 
an individual can resemble either father or mother in any 
feature of its structure. 




Fig. 196. — The pollination of Vallisneria 



spiralis, a common water plant ; 
Kerner.) 



(After 



Ch. VI, 4] NATURE OF FERTILIZATION 



285 



There comes, however, a break in the regularity of this 
chromosome division. It occurs when the adult plant is 
forming its own sex cells (pollen and embryo sac). At this 
time one cell division, called the reduction division, yields 
to the new cells only half the number of chromosomes which 
had prevailed through the body. 
The functional significance of the 
reduction is perfectly clear, for if 
the uniting sex cells contained the 
full number of chromosomes, ob- 
viously the number would double 
in every generation, to their ulti- 
mate enormous multiplication, 
whereas by the reduction division 
the number is kept constant. The 
details of the reduction division are 
complicated and not wholly under- 
stood, but it occurs in such a way 
as to give each sex cell one complete 
set of chromosomes instead of the 
double set which all body cells pos- 
sess. These chromosomes, however, 
(and this is a point of great con- 
sequence in heredity), do not repre- 
sent individual chromosomes which 
occurred in the body cells, but are 
reconstructed from them in such a 




Fig. 197. — Flowers of 
the Hazel (Corylus Avellana) ; 
X |. The staminate flowers 
hang in two large catkins, 
and the pistillate flower 

way as to include some material stands near the top of the 
from the father set and some from 



stem. (From Balfour.) 



the mother set, in combinations which apparently are due 
only to chance, and never the same in any two. This mat- 
ter is illustrated by the diagram of Figure 219, and is 
apparently identical in every respect in plants and animals. 

Thus the principal consequence of fertilization seems to 
consist in the introduction of complete sets of paternal and 
maternal heredity-carrying chromosomes into every cell 



286 



A TEXTBOOK OF BOTANY 



[Ch. VI, 5 



throughout the body of each individual, though whether 
this is the functional aim, or only an incidental result of 
fertihzation, is uncertain. 

What now is the significance of fertilization, or, in other 
words, of sexual reproduction ? Fertilization is by no means 
essential to reproduction, since many kinds of low organisms 
lack it, while even the higher plants possess many methods 
of purely vegetative, or asexual, reproduction, as a later 

section will make clear. 
The asexual method is 
thus not only possible, 
but it is actually much 
simpler and easier of ac- 
complishment than the 
sexual. Why then the 
overwhelming predomi- 
nance of the sexual 
method ? Unfortunately 
we have not yet any cer- 
tain knowledge upon this 
point. It has commonly 
been believed that indi- 
viduals produced by fer- 
tilization are more vari- 
able than those produced 
by asexual methods, and 
that such variability gives advantage in competitive evo- 
lution. Yet some investigators hold other views, and the 
matter is one on which we must await further evidence. 




198. — Various forms of pollen 
magnified. 



Fig. 
grains 

The 3 three-lobed grains in the row on 
the left are Pine, the two lateral lobes 
being air-filled bladders. The roughened 
forms are mostly carried by insects, in the 
hairy bodies of which the various projec- 
tions become caught. (Reduced from 
Kern«r.) 



5. The Methods and Meaning of Cross-Pollination 

It was said in a preceding section that the transfer of pollen 
from the anthers to the stigmas of flowers is effected, as a 
rule, not by action of the plant itself, but by some external 
agency, notably wind and insects. The matter, however, 
goes farther than this, for the floral arrangements are such 



Ch. VI, 5] METHODS OF CROSS-POLLINATION 



287 



that the transfer is usually not between anther and stigma 
within the same flower, but between anthers and stigmas of 
different flowers, — and oftener than not between different 
plants, of course of the same species. This cross-pollina- 
tion, as it is called in contrast with close-pollination 
within the same flower, is a matter of much botanical conse- 
quence. 

In the simpler kinds of 
plants, e.g. the low Algse 
(page 12), the physiological 
equivalent of cross-pollina- 
tion results incidentally from 
the fact that the free-swim- 
ming sexual cells (or gam- 
etes), 'usually cast out into 
the water by many plants of 
one kind together, inter- 
mingle and unite promis- 
cuously. Of course in such 
cases a union may result be- 
tween cells from the same 
plant, as doubtless often 
occurs, though in higher forms 
there is reason to believe that 
chemical attractions and re- 
pulsions between the gametes 
compel crosses between differ- 
ent plants. In the Ferns^ and some other plants having 
free-swimming male ceUs, a cross is insured through the 
fact that the sperm and egg cells produced by the same 
plant are not usually mature at the same time. 

Some flowering plants are pollinated on the water, no- 
tably our submerged pond weeds, eel-grasses, etc. A typical 
case is the fresh-water eel-grass, Vallisneria spiralis (Fig. 196). 
Growing in masses together, these plants produce pistil- 
late flowers which float by long stalks at the surface, 




Fig. 199. — Flower of Iris, cut ver- 
tically. 

The stamen, somewhat to the 
right of the center, is under the style ; 
the stigma is the upper side of the 
small projection from the latter. 
(From Le Maout and Decaisne.) 



288 



A TEXTBOOK OF BOTANY 



[Ch. VI, 5 




and staminate flowerS; which become loosened and rise to the 
surface ; here they float about until they touch a stigma of 
the floating pistil, when fertihzation is effected in the usual 
way. Afterwards the pistil is drawn deep under water, and 
held there during ripening by the spiral coiling of the stalk. 
Such water-pollinated flowers are mostly so inconspicuous 
as not to be popularly recognized as flowers at all. They 
exhibit, indeed, no other floral characteristics than the 
possession of the comparatively obscure, even though vastly 
important, stamens and pistils, although some kinds possess 
rudimentary petals and sepals as relics of their evolutionary 

history. Some water 
plants, however, no- 
tably the Water-lilies, 
retain their showy 
flowers pollinated by 
insects. 

In the flowering land 
plants the simplest 
agency of pollination 
is the wind. Wind- 
pollination occurs in 
most of our trees, e.g. 
Elms, Birches, Oaks, Pines; in some shrubs, e.g. Alders; 
and in a few herbs, — notably Grasses, including the 
Corn. A typical case is the Hazel (Fig. 197), in which 
the long dangling catkins are clusters of staminate flowers, 
while the pistillate flowers are so few and inconspicuous as 
to require special search even by the trained eye of the bot- 
anist, and would hardly be recognizable at all were it not for 
the relatively prominent feathery stigmas. When ripe, the 
pollen, easily shaken from the catkins by a touch, is wafted 
about on the lightest breezes, so that some of it comes into 
contact with the stigmas, though of course an overwhelming 
preponderance is wasted. This case illustrates the typical, 
though not invariable, features of wind-pollinated flowers, 



Fig, 200. — Dichogamous flower of Scrophu- 
laria nodosa; X 2. 

Left, a flower with ripe stigma but unripe 
stamens, shown better in the section (middle 
figure) ; right, a flower with ripe stamens 
but withered stigma. (After Warming.) 



Ch. VI, 5] METHODS OF CROSS-POLLINATION 



289 



which are these : first, they are inconspicuous, for they 
lack colored corolla, odor, and nectar, such prominence as 
they possess being due simply to their abundance, or to the 
yellow color of their anthers. Second, the male blossoms 
far exceed in number the female, in obvious correlation with 
the wastefulness of this method. Third, the stigmas are 
large, often branching-feathery, thus spreading a greater net 
for the drifting pollen. Fourth, the flowers are borne in 
ways to insure free passage of the pollen without inter- 




FiG. 201. — Dimorphic flowers of Primrose ; X 2. 
Pollen from the upper stamens will develop on an upper stigma, and from 
lower stamens on a lower stigma, but not other ways. (From Bailey.) 



ference by presence of leaves. Thus the flowers unfold 
before the foliage in the spring, as with most of our trees, 
or else they are lifted beyond or above the leaves, as with 
Pines and Grasses. Fifth, the pollen is found, on micro- 
scopical examination, to be light in weight and to spread large 
surface, which is even increased, as in Pines, by extension 
into large empty bladders (Fig. 198). Sixth, the stamens and 
pistils are commonly borne in separate flowers, often upon 
different plants, thus preventing close- and insuring cross- 
pollination. While these characters are typical in wind- 
pollinated flowers, they are not invariable ; for wind-polli- 



290 



A TEXTBOOK OF BOTANY 



[Ch. VI, 5 



nation merges over gradually to insect-pollination, the floral 
structure being also intermediate, as some Maples and Wil- 
lows illustrate. 

The most prominent of the agencies of cross-pollination, 
however, are insects, to the utilization of which the most 
typical flowers are fitted. The characteristics of insect- 
pollinated flowers are 



these. First, the flower 
is conspicuous to the 
eye, through a showy 
corolla usually colored 
in contrast to the back- 
ground and set prom- 
inently forth in the 
light; and often its 
presence is indicated 
also by strong odor. 
Second, nectar is com- 
monly present in the 
base of the flower, and 
when absent, is replaced 
by more abundant 
pollen which some in- 
sects particularly de- 
sire. Third, stamens 
and pistils are usually 
present in the same 
flowers. Fourth, the 
shape of the flower is 
such that an insect in probing for nectar leaves some 
pollen on the stigma, and receives some by contact with 
the anthers. Fifth, the pollen grains are commonly angu- 
lar, roughened, or adhesive (Fig. 198). Observation, 
supplemented by experiment, proves that insects visit 
flowers for the nectar, their usual food : that they are 
guided in general to the flowers by the bright spots of 




Fig. 202, — The cleistogamous flowers of 
the common Blue Violet ; reduced. 

The cleistogamous flowers are on run- 
ning stalks on the ground, o is a small 
flower, and h a pod containing seeds. Ordi- 
nary flowers, of natural size, on the right. 
(From Bailey.) 



Ch. VI, 5] METHODS OF CROSS-POLLINATION 



291 



contrasting color, although odor is often more potent in this 
respect ; and that in probing about in their active way for the 
nectar, their hairy bodies brush pollen upon the stigmas and 
receive a new supply from the anthers. All of these matters 
the student can confirm for himself in any garden in summer. 
At first sight it would seem that insects must effect close- 
rather than cross -pollination, but such in fact is not the 
case. Any one can see that insects in gathering nectar 
usually visit flower after flower and plant after plant of the 
same kind as long as 
these are plentiful; 
and since their 
bodies possess a large 
pollen-carrying ca- 
pacity, it must usu- 
ally happen that any 
given stigma, even if 
pollinated from the 
same flower, be- 
comes also pol- 
linated by other 
plants from the 
abundant mixture 
the body of the 




Fig. 203. — Flower of Salvia pratensis, pol- 
linated by a Bee ; X j. 

Right, the flower ready for pollination, show- 
ing position of stigma and stamens. The stigma 
is touched by the insect in entering, and later 
the stamens are brought down on its body by 
operation of a hinged-lever arrangement. (From 
Wiesner.) 



on 



is found to be the 
poUen which effects 
pollen tubes being 



insect. In such cases, as a rule, it 
foreign and not the flower's own 
the fertilization, the growth of the 
more rapid in the former than in 
the latter case. Flowers, indeed, are known which are en- 
tirely sterile to their own pollen. Furthermore, in some 
plants close-pollination is prevented by mechanical arrange- 
ments, as with the Iris (Fig. 199), where the stigma is upon 
a shelf struck by the insect when entering but not when 
leaving the flower. Still more common are the cases in 
which the stamens and pistils are not ripe at the same time, 
as with Scrophularia, an arrangement called dichogamy 
(Fig. 200). And there are plants, of which the Primroses 



292 



A TEXTBOOK OF BOTAKY 



[Ch. VI, 5 



are the most prominent, in which pollen and stigmas are of 
two reciprocally corresponding kinds, though the two never 
occur in the same flower, an arrangement called dimorphism 
(Fig. 201). These and other arrangements bring it to pass 
that cross-pollination is in fact the rule in showy flowers. It 
is not, however, invariable, for with many kinds of flowers, 
especially of the simpler sorts, close-pollination is perfectly 




Fig. 204. — Cypripedium Calceolus. 
The insect can enter only by the front or upper opening, and leave only 
by a side opening ; thus it must pass in succession the stigma and anthers. 
(From Kerner.) 

possible, and is actually self-effected, if no foreign pollen 
be present. Still further, there exist a few flowers, of which 
some Violets are examples (Fig. 202), in which self-pollination 
is not only the rule, but the invariable rule ; for the flowers 
never open and the anthers shed the pollen directly upon 
the stigmas, an arrangement called cleistogamy. All such 
plants, however, possess also showy flowers, cross-pollinated 
in the usual way. 



Ch. VI, 5] METHODS OF CROSS-POLLINATION 



293 




Not only does the structure of the typical showy flower 
exhibit remarkable fitness to cross-pollination by insects, 
but this fitness is carried to degrees which have evoked the 
wonder and admiration of long generations of loving ob- 
servers of plants. The simplest condition is found in regular 
flowers like Buttercups or Apple blossoms, where almost 
any kind of insect may equally well alight in the shallow 
basin, and, busily seeking the 
nectar, effect pollination. This 
is likewise the case with the 
Compositse, — the Dandelions and 
Daisies and Sunflowers, and that 
sort. In irregular flowers, such 
as the Larkspur and Mints (Fig. 
203), the arrangements are such 
that only Bees and like insects 
can reach the nectar in the elon- 
gated spurs or tubes; and those 
are the principal insects which 
visit such flowers. In these 
flowers, as elsewhere, the me- 
chanical arrangements are such 
that the visiting insect must take 
a path which insures cross-pol- 
lination. In some Orchids, espe- 
cially the Lady's Slipper (Fig.204) , 
the insect has to enter the flower 
by one opening which the stigma guards, and leave by another 
over which hangs an anther. In Orchids, indeed, the fitting 
of floral form to insect shape and habit has become wonder- 
fully exact, so that in some cases only a single species of 
insect can pollinate the flower, the adjustment between the 
two being carried remarkably into details. These, how- 
ever, are but few of the great variety of arrangements pre- 
sented in this relation between flowers and insects, which 
include even a case of deliberate and purposeful pollination 



Fig. 205. — A flower of Yucca 
Whipplei, being pollinated by a 
Pronuba moth; X {. 

The insect deliberately col- 
lects pollen from one flower, car- 
ries it to the stigma of another, 
and there presses it securely 
down. It then lays an egg in 
the ovary of that flower, and its 
larva feeds on some of the seeds, 
which would not develop without 
the pollination. (From Kerner, 
after work by Trelease.) 



294 



A TEXTBOOK OF BOTANY 



[Ch. VI, 5 



by the insect (Fig. 205). All of these matters are described 
in detail in several works devoted to the subject. The 
student will find that to the original investigators, these re- 
markable adjustments between flowers and insects seem most 
reasonably explained as result of a gradual process of adapta- 
tion of one to the other during the course of their evolution. 
Insect-pollination prevails far more widely than any other 
method, and moreover is characteristic of the highest kinds 
of plants. A reason for its superiority over wind-polhnation 
consists obviously in its greater economy and efficiency, for 

in the one case the 
pollen is simply cast 
forth and its access to 
a stigma left to chance, 
while in the other the 
pollen is carried di- 
rectly from anthers to 
stigmas. A phase of 
this economy — mak- 
ing the most, so to 
speak, of insect visits 
— explains the pres- 
ence of stamens and 
pistils in the same 
flowers. 

While insects are overwhelmingly the most important, 
they are not the only animal cross-pollinators of flowers. 
Some kinds of large bright flowers are regularly pollinated 
by small nectar-eating birds, especially humming-birds 
(Fig. 206) ; and others, growing close to the ground, are 
pollinated by snails, which are attracted by a succulent tis- 
sue formed in the spike among the flowers. But insects, 
from their combination of small size, active habits, and 
nectar diet, make the most effective cross-pollinators. 

As with other plant organs, flowers have not only their 
primary function, which they perform as their contribution 




Fig. 206. — Marcgravia nepenthoides, polli- 
nated by humming birds ; much reduced. 

In the pouched nectaries below the flower, 
there is secreted abundant nectar, in probing 
for which the hovering birds bring their heads 
successively against the flowers. (After H. 
Miiller.) 



Ch. VI, 5] METHODS OF CROSS-POLLINATION 



295 



to the economy of the plant as a whole, but also certain sup- 
plementary functions essential to their own nutrition or 
safety. Thus pollen is commonly hable to injury by water, 
i.e. rain, through osmotic absorption, as earlier shown 
(page 234) ; but many flowers are completely inverted, thus 
shedding the rain, as in 
Columbine (Fig. 207), or 
the petals overarch the 
stamens, or scales and 
hairs prevent access of 
raindrops, or other ar- 
rangements occur. Also 
the nectar is attractive 
to insects too small, or 
unadapted by habit, to 
effect pollination, no- 
tably ants, against which 
there must needs be pro- 
tection ; and a good many 
corresponding adapta- 
tions have been claimed 
in flowers, — the closed 
throat of the Snapdragon, 
which bees can open but 
ants cannot, the adhesive 
glands on the calyx of 
Plumbago, the hairs in 
the throat of many flowers, 
and a good many others, 
for further accounts of 
which the student must turn to the special works. Of 
course, as with other organs, various hereditary and struc- 
tural factors also enter into the construction of particular 
flowers, which therefore can by no means be explained in 
detail upon the basis of adaptation to insect-pollination 
alone. 




Fig. 207. — The Columbine, Aquilegia 
canadensis, showing the inverted position 
of the flowers. (From Bailey.) 



296 



A TEXTBOOK OF BOTANY 



[Ch. VI, 5 



Not only a structural but also a physiological fitness to 
the accompUshment of pollination occurs in flowers. Thus, 

most flowers are very strongly 
phototropic, as one can see very 
easily on any sunny bank or in 
borders near buildingS; and as can 
be proven very strildngly by ex- 
periment (Fig. 208). Thus they 
are brought, and their faces set, in 
the most conspicuous positions. 
Again, many flowers, especially 
those which present a special 
alighting position to insects, are 
very strongly geotropic (diageo- 
tropic), and thus, no matter what 
accident may befall the flower 
stalk, the individual blossoms keep 
their positions for the visiting in- 
sect (Fig. 209). The tubes and 
coronas of Narcissus and Daffodils 
are kept thus in their horizontal 
positions, as can be proven by tying 
a young flower stalk down hori- 
zontally. Again, many flowers do 
not open before rain, or else close 
on its approach, and some few 
flower-clusters turn upside down, 
seemingly in adaptation against 
damage to the pollen by water 
Fig. 208. -a cluster of (page 234). The stimuU Concerned 

Belmowers, set m one-sided ^a o ^ 

light, showing phototropism of are not in all cases clear, nor are 
the flowers (Drawn from a ^Yie Weather predictions always 

photograph.) . . 

accurate, though in this they per- 
haps succeed as often as our human forecasters with all their 
exact instruments. Also, various movements of stamens 
occur, partly effected mechanically, as in Mountain Laurel 




Ch. VI, 5] METHODS OF CROSS-POLLINATION 



297 



(Kalmia), partly irritably, as in Barberry, and diversely in 
some Orchids, though the exact significance of these and 
other Uke phenomena is still matter of debate. 




Fig, 209. — Clusters of Larkspur, showing the diageotropism of the flowers. 
The tip of the larger cluster was tied down at an early stage of its devel- 
opment. (Drawn from a photograph.) 

There remains one leading question : Why these elaborate 
arrangements for cross-polHnation when close-pollination is 



I 



298 A TEXTBOOK OF BOTANY [Ch. VI, 6 

so much simpler, as cleistogamous flowers attest ? There 
is good evidence, based on direct experiments by Darwin, 
that more and stronger offspring are produced by cross- 
polhnation than by close-, meaning a cross between different 
plants, for a cross between different flowers on the same 
plant is little if any better than close-pollination. Thus an 
advantage must inhere in cross- over close-pollination, which 
means of course in cross- over close-fertilization, pollination 
being only a preliminary mechanical step to fertilization. 
This superiority, in light of the well-known evolutionary 
principle of the survival of the fittest, amply explains the 
prevalence of cross-fertilization in plants, and its exclusive 
use in the higher animals. But why is cross- superior to close- 
fertilization ? As to this, there is still much doubt, but we 
can gain some light by translating the matter into terms 
of the chromosome mechanism. Close-fertilization must 
necessarily give to the offspring two sets of chromosomes 
identically alike, precisely as in case of asexual reproduction 
(page 300), while cross -pollination gives to the offspring two 
sets of chromosomes which differ somewhat. Hence cross- 
fertilization has the same superiority over close- that close- 
has over none ; or to put the matter in another way, cross- 
fertilization is apparently necessary in order to realize the 
full benefits of -fertilization. As to the advantages of fer- 
tilization over asexual reproduction we have already spoken 
(page 286). One must regret that such fundamentally im- 
portant matters cannot as yet be satisfactorily explained, 
but they cannot. The future, however, will surely yield 
their interpretation. 

6. Methods of Asexual Reproduction 

While most reproduction in plants involves fertilization 
and sex, there is much that is purely asexual. Sexual and 
asexual methods are by no means mutually exclusive, but 
commonly exist together in the same plants, either side by 
side or in a form of alternation. 



Ch. VI, 61 ASEXUAL REPRODUCTION 299 

The only plants which reproduce exclusively by asexual 
methods, so far as known, are the very simplest Algse and 
the Bacteria. Their single-celled bodies divide across into 
two ; the halves grow to full size and fall apart (Fig. 210) ; 
and thus we have asexual reproduction by division. One 
might think also that the higher Fungi should be included 
in the asexual category; but newer studies seem to be 
showing that these plants possess a form of fertilization. 

In addition to division, the asexual methods include 
specialized vegetative bodies, potential vegetative parts, and 
asexual spores. 

1. Specialized vegetative bodies. These are mostly of the 
nature of separable buds, formed on thallus or shoot and 
later set free, when they 
grow directly to new 
plants. Such bodies are 
found in Algse and Fungi, 
especially the Lichens, 

and reach fuller devel- Fig. 210. — Asexual reproduction, by 

Opment in the higher division, of Pleurococcus, a green Alga 

^ ^ wnicn grows upon tree trunks ; much 

groups. Thus, some magnified. 

Liverworts produce in , ^he smaller cells thus formed soon grow 

^ to lull size. 

special cups on the thallus 

many sjnnmetrical flat bodies called gemmae (Fig. 211), which, 
washed out or blown to some distance, produce there new 
plants. Some Ferns produce on the margins of their fronds 
little buds, which sprout and continue their growth when 
the leaves are dropped. Identically the same feature is 
found in the Life-plant, or Bryophyllum (Fig. 43), which 
has buds on the margins of its leaves, and thus can produce 
new plants when the leaves fall on the ground. Essentially 
the same principle is involved in the formation of Httle 
plants at the ends of the runners in the Strawberry, as earlier 
described (page 189). Some waterweeds form compact win- 
ter buds, which separate and float away to start new plants 
in the spring. The larger Lilies produce in the axils of their 




300 



A TEXTBOOK OF BOTA>v[Y 



[Ch. VI, 6 




Fig. 211. — The thallus of a Liver- 
wort, Marchantia; natural size. 

The cups bear small vegetative ge:n- 
mse, of which one is shown enlarged. 
(From Kerner.) 



leaves little seed-like bodies (Fig. 212), which really are com- 
pact buds though they separate and give rise to new plants. 
Similar in nature, albeit very different" in ap- 
pearance, are the little bulblets which so many 
liliaceous plants produce as 
outgrowths from their main 
bulbs. This is a great 
profit to our gardening, for 
the possibility of our bulb 
beds and borders depends 
upon the existence of 'these 
bulblets, which are simply 
separable buds. 

2. Potential vegetative 
parts. Some plant parts 
not specialized for repro- 
duction can yet serve in- 
cidentally that function. Thus the brittle twigs of Willows, 
if blown or drifted to suitable places, take root and grow to 
new trees. Many kinds of creeping root- 
stocks, or even roots, will produce new 
plants when severed, as exemplified by 
the pertinacious Couch Grass of our gar- 
dens. The most important phase of such 
reproduction, however, is found in the 
power possessed by many plants to strike 
root from cuttings placed in the ground, 
whereby they produce full and perfect 
plants, even though they never reproduce 
naturally by this method ; and many plants 
which will not strike root from ordinary 
cuttings can yet be made to do so by 
devices well known to skilled gardeners. 
This potentiality of vegetative reproduc- 
tion, rarely or never exercised in most of these plants, is 
rich in consequences for practical gardening. 




Fig. 212. — A 
seed-like separable 
bud on the stem 
of a Lily. (From 
Bailey.) 



Ch. VI, 6] 



ASEXUAL RJ^PRODUCTION 



301 



3. Asexual spores. A spore is typically a single-celled 
body which can grow directly into a new plant. Many of 
the simpler Algae throw out into the water tiny spores which 
swim freely about by aid of small cilia, and hence are called 
ZOOSPORES (Fig. 213) ; they resemble closely the gametes 
later described (page 303), but grow without any fusion 
into new plants. Higher Algae also produce such spores, 
which are thrown off to 
drift with the currents, 
as will be described in 
Part II of this book. 
Most of the Fungi pro- 
duce asexual wind-scat- 
tered spores, usually in 
very great number, and 
minute as the dust, 
either on the gills, as 
with Mushrooms, or in 
special long-stalked 
spore cases, as in Molds 
(Fig. 214), or in other 
equivalent ways . 
Liverworts and Mosses 
produce spores in their 
capsules, and Ferns in 
the ''fruit dots" on the 
under sides of their 
fronds, as will later 
more fully appear. In all of these groups except the Fungi, 
and even obscurely in them, occurs also a sexual reproduc- 
tion with fertilization, as already described, while moreover 
there exists usually, and perhaps always, a marked alter- 
nation between the sexual and asexual methods. This 
latter subject merges into the so-called alternation of 
GENERATIONS, a matter of very great morphological inter- 
est, to which we shall return in Part 11. 




Fig. 213. — Zoospores, asexual unicel- 
lular reproductive bodies, of Algse ; highly 
magnified. 

The forms are typical. All swim by action 
of the hair-like cilia, — towards the cilia. 



302 



A TEXTBOOK OF BOTANY 



[Ch. VI, 7 



A very special and rare form of asexual reproduction is 
found in parthenogenesis, i.e. the development of an egg 
cell to a plant without fertilization. This is known in 
relatively few plants, which include especially the Composite 
family, though it is frequent in the reproduction of Insects. 
Closely related thereto are cases of polyembryony, in 
which cells of the embryo sac other than the egg cell de- 
velop into apparent embryos, 
which, ho-^ever, are really 
vegetative structures of the 
nature of branches. Both 
parthenogenesis and poly- 
embryony are too rare to 
play any appreciable part in 
plant reproduction as a whole, 
and have their chief interest 
as showing the plasticity of 
natural processes. 

A question must arise at 
this point as to whether any 
differences exist between the 
sexually and asexually pro- 
duced offspring of the same 
plant. It seems clear that 
in general the asexually pro- 
duced individuals are less 
variable in their characters 
than the sexually produced, 
although there are notable exceptions. At all events it is cus- 
tomary among gardeners to employ asexual propagation when 
they wish to retain a variety true to some valued character, but 
to use sexual or seed reproduction when trying for novelties. 

7. The Origin and Significance of Sex 

In the preceding sections, the two fusing sexual nuclei, 
the male and the female, have been treated as functionally 




Fig. 214. — The spore cases and 
spores of a common Mold ; X 38. 
(From Strasburger.) 



Ch. VI, 7] 



SIGNIFICANCE OF SEX 



303 



alike and contributing the same in kind to the offspring. 
Such is the conclusion indicated by modern research. If, 
now, the two sexes thus contribute equally to the constitu- 
tion of the offspring, where lies the essential difference be- 
tween them, or, more exactly, why does sex exist at all ? 

For light upon this question we turn to the sexual methods 
and structures presented by the different groups of existent 
plants, from the lowest to the highest. These will be found 
described in full in Part II of this book, but for our present 
purposes may be sum- 
marized as follows. 

I. There are plants of 
simple kinds, notably 
Bacteria and some low 
Algae, which reproduce 
solely by division, that is, 
the single cells constitut- 
ing the adults simply di- 
vide across and grow to 
full size (Fig. 210), pre- 
cisely as do meristematic 
cells already described in 
the higher plants (page 

299) . Here is represented a stage of reproduction in which 
there is neither fertilization nor sex. 

II. There are several known Algae, of grade somewhat 
higher than those just mentioned, in which the plants are 
aU alike, and produce small reproductive cells called gametes, 
likewise all alike, and provided with swimming appendages. 
These gametes are thrown out into the water, where, swim- 
ming freely about, they come together at haphazard and 
fuse, uniting their nuclei, quite in the manner of the fer- 
tilization of the higher plants (Fig. 215) ; and from the re- 
sulting cell a new plant develops. Here is evidently repre- 
sented a stage in which fertilization occurs, but without 
any difference between the sexes. 




Fig. 215. — Gametes of the simple Alga 
Protosiphon, in process of fusion ; highly 
magnified. On the right a complete 

"zygote." 



304 



A TEXTBOOK OF BOTANY 



[Ch. VI, 7 



III. The Rockweeds, the common brown seaweeds so 
prominent on sea coasts at low tide, and some other Algae of 
higher grade than those mentioned under II, produce two 
kinds of reproductive cells, one relatively large, round, and 
without swimming appendages, the other small, elongated, 
and adapted to swim freely (Fig. 216). Both kinds when 
ripe are thrown into the water, where the large cells float 
passively about while the small cells swim to them and fuse 
with them, quite in the manner of fertilization in the higher 
plants; and this fertilized cell grows into a new plant. 

We call the larger the egg cell, 
or EGG, and recognize it as 
female, and the smaller the 

SPERM CELL Or SPERMATOZOID, 

and recognize it as male ; and 
herein we have a clear case of 
the existence of sex. Consid- 
ering, now, the nature of the 
differences between the two sex 
cells, it is evident that the egg 
cell owes its great size to the 
large supply of food it contains, 
this food being used in the de- 
velopment of the new plant un- 
til it can make its own supply ; 
and since it is thus large and clumsy, so to speak, its capacity 
for free locomotion is diminished, and even the attempt is 
abandoned. The sperm cell, on the other hand, consists 
of little more than a nucleus, with only enough cytoplasm to 
construct an efficient swimming apparatus. Here, as in 
the higher plants, the two nuclei appear to contribute through 
their chromosomes exactly alike to the offspring, and it seems 
clear that the difference between the two cells consists in a 
division of labor with respect to two subsidiary features 
of reproduction, viz. the bringing of the sex cells together, 
and the provision of food for the resultant offspring, — one 




Fig. 216. — An egg cell of 
Rockweed, surrounded by sperm 
cells, one of which enters and effects 
fertilization ; X 500. (Redrawn 
from L. Kny.) 



Ch. VI, 7] 



SIGNIFICANCE OF SEX 



305 



m 



cell assuming wholly the one function, and 
the other the other. No differences occur 
in the plants which produce these cells, 
excepting in the parts immediately con- 
nected with the formation of cells of such 
different sizes. Thus we have a stage in 
which there is a clear distinction of sex, but 
only in the sexual cells themselves, and it 
arises not from any fundamental matter of 
difference in contribution to the constitution 
of the offspring, but in a secondary matter 
of division of labor in connection with the 
mechanism of fertilization, and the nutrition 
of the resultant embryo. 

IV. The higher, or Red, Algae 
have a complicated reproduction 
under which we can recognize the 
essential fact that the egg cell, 
naked as in the lower kinds, remains 
permanently attached to the parent 
plant, upon which it is fertilized by 
a much smaller floating sperm cell, 
and from which the resultant 
growth is supplied with food (Fig. 
217). Thus we have a stage, not, 
it is true, exactly represented in 
living forms, but presumably once 
occurring in kinds now extinct, 
wherein the egg cell remains at- 
tached to the parent plant, on 
which it is fertilized and by which 
the resultant equivalent of an em- 
bryo is supplied with food. 

V. The stage just described is 
the highest attained by the Algae. (After L. Kny.) 

In the simplest land plants, the Bryophytes and Ferns, 




Fig. 217. — The egg cell, 
attached to a fragment of 
frond, of Nemalion multi- 
fidum, a seaweed ; X 700. 
Extending from the egg cell 
is the long-projecting " tri- 
chogyne," adapted to receive 
the small floating sperm cells, 
of which two are attached. 



306 



A TEXTBOOK OF BOTANY 



[Ch. VI, 7 



the egg cell is no longer naked, but, in obvious correla- 
tion with the danger which would attend the exposure of 
its delicate, succulent substance to dry air, is inclosed 
within a protective cover, so constructed that when the egg 
cell is ready for fertilization and the surrounding conditions 
are favorable, the cover opens, and not only permits, but 
facilitates, the access of the free-swimming sperm cell, which 




Fig. 218. — Sexual reproduction of a Fern ; x 240. 
The structures occur on the under side of the sexual or prothallus stage, 
close to the ground. On the left, a section of the prothallus in which the 
egg cell is buried and covered by the tubular "archegonium." On the 
right, the free-swimming sperm cells escaping from the "antheridium." 
When the sex cells are ripe, the access of water causes both structures to 
open ; the archegonium releases into the water a substance (malic acid) 
very attractive to the sperm cells, which swim towards it, and enter the tube, 
when the first to reach the egg cell fuses therewith and effects fertilization. 
(After L. Kny.) 



itself develops in a special structure (Fig. 218) ; and then the 
developing embryo is supplied with food by the parent 
plant. Here is evidently represented still another stage in 
the evolution of sex, in which have been developed, above 
the earlier differences, special and different structures, which 
protect the sex cells in ways to facilitate the access of the 
free sperm cell to the fixed egg cell. 

VI. The highest development of sex in plants is repre- 
sented in the construction of the flower, as described in an 



Ch. VI, 7] SIGNIFICANCE OF SEX 307 

earlier section (page 269). Here fertilization is effected, not 
in water by a free-swimming sperm cell, as in all earlier 
stages, but in the air by wind- or insect-carried pollen grains 
from which the pollen tubes carry the sperm cells to the egg 
cells. In correspondence with the dry and exposed sur- 
roundings, the egg cell is deeply buried within the body of 
the parent plant, — within an embryo sac, inside an ovule, 
enclosed by an ovary, while the pollen occurs in closed an- 
thers. Now the mode of transport of the pollen, by external 
agencies, requires that the anthers, with some part of the 
oyary fitted to receive the pollen, be accessible to wind and 
insects; and such is the function of stamens and pistils. 
Accordingly these parts, specially fitted to bring the sex cells 
together, constitute physiologically the sexual organs of the 
plant, even though on morphological grounds this designa- 
tion has been denied them. Here is evidently represented 
still another stage in the evolution of sex, consisting in the 
presence of sexual organs, fitted to effect union of the sex cells. 
VII. In most plants the stamens and pistils are borne 
close together in the same flowers, which are said to be 
PEEFECT (or hermaphrodite). In some cases, however, Uke 
Birches and Oaks, they are borne in separate flowers on the 
same plant, when they are said to be monoecious. In any 
case only the stamens and pistils show structural differences 
connected with the different sexes of the cells they produce, 
and the plant itself shows no trace of sex. In a few kinds of 
plants, however, the staminate and pistillate flowers are 
borne upon separate plants (are dioecious), in which case 
the plants are somewhat naturally, though not quite cor- 
rectly, described as male and female. Ordinarily there is 
no determinable difference, aside from the flowers, between 
such plants, but occasionally, as in Date Palm, some Wil- 
lows, and a few others, there is a marked difference in as- 
pect between staminate and pistillate individuals, thus giving 
a structural basis to the terms male and female as applied 
to plants. Here, however, is the limit reached by plants in 



308 A TEXTBOOK OF BOTANY [Ch. VI, 8 

their sexual differentiation, though the higher animals have 
gone a little farther, for in them the male and female 
sex cells are always borne by different individuals, which 
are distinguished, not only by their very different sexual 
organs, but also by correlated differences in habits, occupa- 
tions, dispositions, aspect, stature, and other visible features. 
Thus, in summary, there runs throughout all sexual dif- 
ferentiation the one constant thread of the fusion of the 
two sex cells, which brings together the parental chromo- 
somes in equal contribution to the constitution of the off- 
spring. It is only the mechanisms subsidiary thereto which 
vary. These mechanisms originate in a way to imply that 
the sexes were originally alike, and the differences between 
them arose through a division of labor, at first between the 
sex cells and later between the individuals which produce 
them, in connection with two subsidiary features of sexual 
reproduction, — viz. effecting the union of the sex cells, and 
nourishing (and later protecting) the embryonic offspring. 
Even in the highest plants and animals, sex seems to mean 
no more than this difference, developed to such a degree as 
to produce structures, organs, and even individuals, fitted 
to the respective parts taken by the sex cells. It is indeed 
possible that other factors are also involved in the result, 
but if so, they are obscure, while these are obvious. 

8. Heredity, Variation, and Evolution 

The matters considered in the preceding sections lead 
naturally to others expressed in the title of this section. 
They are largely of theoretical, though very fundamental 
nature. Although in the past largely speculative in treat- 
ment they are now the subject of profound experimental 
researches, the conclusions of which apply equally to plants 
and animals. 

Heredity is the resemblance of an individual to its an- 
cestors. Variation is the difference between an individual 
and its ancestors. It is easy to see how, granting the chro- 



Ch. VI, 8] HEREDITY AND VARIATION 309 

nlosome mechanism earlier described, heredity should oc- 
cur. Indeed, on this basis, offspring should repeat their 
ancestors exactly, and the scheme leaves no room for vari- 
ation at all. 

The student will note the phrase ''like its ancestors," 
not ''like its parents.'' It is a matter of popular knowledge 
that family characteristics often skip a generation, or several 
for that matter ; and children thus show features of their 
grandparents intermingled with those of their parents. 
Our knowledge of this subject is now firmly grounded, thanks 
to the labors of Mendel and his many modern successors in 
experimental biology. As a result it seems clear that the 
characters or features which make up an individual, and 
which are built by its cytoplasm under control of its chromo- 
somes, are not indefinite in number and kind, as popularly 
imagined, but are definite in both respects. In other words, 
an individual consists of a definite, though great, number 
of ultimate irresolvable unit characters, of which it forms 
a kind of mosaic. Furthermore, each such unit character 
is apparently represented in the chromosomes of all of 
the cells by some kind of determiner which controls the 
construction of that character by the cytoplasm, though 
whether this determiner be some material carrier, some kind 
of register, some form of model, some type of enzyme, or 
some other entity, is not known. Accordingly, the ferti- 
lized egg cell, and every body cell formed therefrom, having 
its two sets of chromosomes, must contain two sets of all 
the determiners necessary to construct that kind of organ- 
ism ; or in other words every kind of character of an organism 
is represented in duplicate in every one of its body cells, one 
determiner being contributed by each parent (see the dia- 
grammatic Fig. 219). Now arises the question : How do these 
duplicates behave with respect to one another during the 
development of the cell, and what determines which one is 
to direct the cytoplasmic construction, and thus determine 
the character, in any particular case ? On this matter Men- 



310 



A TEXTBOOK OF BOTANY [Ch. VI, 8 









Fig. 219. — A diagram to illustrate the principle of the chromosome 
mechanism of heredity. 

The triangular masses of cells are adult individual plants, or animals, 
male and female, developed from the parental germ cells shown below, and 
forming above their own germ cells, which are uniting in pairs into fertilized 
egg cells. In the nuclei of the individuals are the chromosomes, reduced 
for simplicity to two, and composed of determiners, reduced for simplicity 
to four, a black determiner being assumed to be always dominant to a 
white one. For example, we may take a triangle to mean height of stem, 
black meaning taller and white shorter ; circle, color of corolla, black darker, 
white lighter ; square, shape of leaf, black longer, white rounder ; diamond, 
texture of stem, black rougher, white smoother. Thus the two individuals 
would be taller, longer-leaved, darker-flowered, rougher-stemmed, though 
having both the capacity to transmit the other qualities, as shown in 
their germ cells. 

Two such individuals as here pictured, being externally alike though 
differently constituted in their chromosomes, are described in the technical 
language of genetics as phenotypically identical but genotypically different ; 
and, having both dominant and recessive determiners, are heterozygous for 
all characters. They can, however, as the diagram shows, produce offspring 
which contain only the dominant or the recessive determiners for certain 
characters, that is, are homozygous for those characters. 



Ch. VI, 8] HEREDITY AND VARIATION 311 

del was the first to obtain exact knowledge, which has been 
confirmed and greatly extended by others. Using different 
varieties as parents, he was able to trace the separate char- 
acters in their hybrid offspring, and thus he discovered that 
the rule in such cases is this, — the matter does not depend 
upon chance, hut one of the two determiners regularly prevails 
over the other (is dominant, in his phrase), and shows its in- 
fluence in the developing cell, while the other is latent (re- 
cessive, in his phrase), and without visible effect. This is 
the way in which parental characters can lie unseen and. 
latent in the body, thus in our common but erroneous phrase 
''skipping a generation." 

There is, however, much more in the subject than this. 
As already explained (page 285), when the adult individual 
forms its own new sex cells, the number of chromosomes, 
and therefore of determiners, is halved by the reduction 
division^ but in such manner as to give to each new sperm 
or egg nucleus one complete set. This set is taken partly 
from the father set and partly from the mother set, the 
combination apparently being made wholly at random, as 
manifest by the fact that the different sexual cells of the 
same individual differ greatly in the make up of their com- 
binations (see Fig. 219). Thus it happens that every sexual 
or germ cell contains a determiner for each character from its 
father or its mother, but never from both, a fact called 
technically ''the purity of the germ cells." It is also true 
that, for any given character, about as many germ cells 
carry the father determiner as carry that of the mother. 
Now if two individuals of the same kind breed together, as 
imagined in our figure, and if the union of the germ cells 
is left simply to chance, as seems to be true, then there 
follows, so far as each single character is concerned, a very 
remarkable and important result, which can most simply 
be described by use of our diagram. Thus, if we center our 
attention upon color of corolla (the circles with black, 
dominant, and white, recessive) ^ we find that four and only 



312 A TEXTBOOK OF BOTANY [Ch; VI, 8 ) 

four modes of fertilization are possible ; a black from a male 
nucleus may unite with a black from a female, or a black from ] 

a male with a white from a female, or a white from a male j 
with a black from a female, or a white from a male with a J 
white from a female. Thus we can have four kinds and i 
only four, of fertilized egg cells, one containing two black '3 
determiners, one containing two white determiners, and 
two containing a black and a white. In other words, the- 
oretically i of all the offspring of this couple will have the 1 
black character only, the white being eliminated entirely from ' 
their bodies and those of all their offspring if they breed only 
with their own kind ; J likewise will have the white character I 
only, the black being eliminated out of them and their off- | 
spring if they breed with their own kind ; and two J's, that | 
is J, of the whole will have the black and white both in their -^ 
own bodies, and can transmit either to their descendants, .1 
although, as black is dominant to white, they will themselves I 
show only the black character, the white being latent or re- 
cessive. Thus of all the offspring f will show the dominant '^^ 
black and } the recessive white, though of the f , f have the 
white latent. The arrangement is represented for a single 
character in Fig. 220. This fact was discovered by Men- 
del in hybrids, but of course is equally true in principle for 
ordinary offspring from parents of the same variety. It has 
been found to hold true very widely, even though not uni- 
versally, in a great many kinds of plants and animals ; and 
it is the central feature of Mendel's Law, now one of the most 
prominent matters in all Biology. 

For the sake of the study of the principle we have re- 
duced our subject to the utmost degree of theoretical sim- 
plicity. In fact, however, matters are never so simple, and 
commonly are vastly complex, in actual Hfe. Thus, the 
law only holds true as an average of high numbers, its oper- 
ation being often obscured by chance with small numbers ; 
characters and determiners are not few in number, but 
many, even to hundreds and thousands ; similar forms are 



Ch. VI, 8] HEREDITY AND VARIATION 313 

not likely to breed together repeatedly unless compelled by 
experiment, though the same result is effected in some 
plants which pollinate themselves ; characters are not passed 
along singly, but commonly a number together in loose 
aggregations ; determiners seem to exert certain influences 
upon one another directly ; and there are yet other compli- 
cations. Hence in Nature the law is not manifest to obser- 
vation, though discoverable by experiment ; but it operates 

o • 



o o o • 

I i' I I I I ' I I nh~i I I ' I I 
oooo ooo» ooo« •••• 

Fig, 220. — Diagram to illustrate Mendel's Law of the segregation of 
characters in heredity, using a single character of Fig. 219. 

If germ cells having the dominant character (black circle) breed with 
others having the recessive character (white circle), then all of their off- 
spring show only the dominant character but carry the recessive character 
latent (black circle with white center) . If these forms breed together, their 
offspring will show the distribution of characters represented in the diagram, 
— one-fourth will be pure dominants and one-fourth pure recessives, while 
the remainder are dominants carrying the recessive character latent. If 
those in this generation breed only with their own kind, the result in the 
next generation is as shown in the diagram ; and thus indefinitely. 

as a steadily working principle which runs as a kind of 
guiding thread through all heredity, while coming to view 
now and then in such phenomena as ''skipping a genera- 
tion," elimination of characters from a race, and other 
less obvious matters. 

Thus, on the basis of our knowledge of the performance of 
the chromosomes in reproduction, in conjunction with 
Mendel's law, heredity must rest upon the transmission of 
determiners which, existing in each species in a certain 
number, are distributed in different combinations in the 



314 A TEXTBOOK OF BOTANY [Ch. VI, 8 

different individuals. Expressed otherwise, and somewhat 
fancifully, individuals are simply temporary kaleidoscopic 
combinations of the various determiners belonging to the 
species, the act of reproduction, especially the reduction di- 
vision and subsequent fusion, providing the new turn of the 
kaleidoscope. 

Thus much for heredity, which means the resemblances of 
individuals to their ancestors. What now of variation, which 
means the differences? The chromosome mechanism ex- 
plains heredity well, but not variation. Indeed the mechan- 
ism seems to leave no room for variation, since by its oper- 
ation all individuals are simply combinations of determiners 
which preexist. Yet variation is as real a fact as heredity, 
for organisms do change with time, as proven by comparison 
of living plants and animals with their fossil ancestors. 

The conception of variation, however, needs definition, 
for some apparent variation is not at all important in evolu- 
tion. Thus, individuals are often strongly altered in their 
development by their conditions of life, — insufficient or 
peculiar food, etc., and also often become altered by self- 
adjustment to the conditions of their immediate surround- 
ings, as we have noted already under various phases of 
irritability. But such changes (called fluctuations) are 
known not to be hereditary, that is, they affect the cyto- 
plasm but not the determiners in the chromosomes. The 
variations (called genetic variations, or mutations), 
which produce hereditary alterations in organisms, must 
affect the determiners, either by interpolating new ones, 
or by altering the character or relations of those already 
present. Yet while such mutational variation undoubtedly 
exists, we have no knowledge as to how it arises or in what 
way it affects the determiners. Indeed the origin of varia- 
tion is the great crucial problem of present-day Biology, 
though it will be settled, and before long, by the experi- 
ments now in progress. It is the watching understand- 
ingly for the answer to such deep questions which gives to 



Ch. VI, 8] HEREDITY AND VARIATION 315 

the study of science its great charm, and it is the chance to 
find the answer one's self which gives to scientific investiga- 
tion its matchless zest. 

That the organisms which now exist on the earth are 
different from those which formerly existed, and that these 
organisms are fitted to the conditions under which they 
live, are two facts which have long been known to scientific 
men, who have explained them in different ways. Thus 
Linnaeus, and most others of the earlier naturalists, be- 
lieved that the new kinds were each suddenly created, and 
in very exact fitness to the surrounding conditions, by an 
omnipotent Creator. This doctrine is known as special 
CREATION. It did not, however, stand the test of advancing 
knowledge, for ample evidence seemed to show that existent 
kinds of organisms have developed out of earlier kinds ; 
and it seemed reasonable to suppose that in course of this 
development the organisms and their parts became adapted 
to their environments. This is the meaning of evolution. 
All modern research has tended to confirm its correctness. 

The fact of evolution is one thing, and the method whereby 
it has come about is another; and the explanation of its 
method has been for a half century the foremost problem 
of philosophical biology. Two great leading solutions have 
been offered for the problem. Lamarck, a French zoologist 
who was active a century ago, argued that the changes which 
are known to occur in individuals, either directly by action of 
the environment or by self-adjustment thereto, are trans- 
mitted to the next generation and there re-appear; and 
that thus a character can be intensified generation after 
generation until a new kind or species results. This is the 
view of the transmission of acquired characters. Trans- 
lated into terms of the chromosome mechanism, it would 
mean that any change in a character of an individual or- 
ganism, which of course affects the cytoplasm of the cells 
concerned, can become registered or represented in some way 
in the determiners in its germ cells. Now of such a result 



316 A TEXTBOOK OF BOTANY [Ch. VI, 8 

there is not only no known evidence, but such evidence as 
we possess seems wholly against its occurrence, with possible 
rare exceptions which hardly affect the general principle. All 
evidence seems to show that while alterations in the deter- 
miners alter the organism, the reverse is not true. 

A second solution, and the most famous, is that of Darwin, 
who was active in his work somewhat over a half century ago. 
He argued that a spontaneous variation of all features of 
organisms is constantly in progress ; that only a few of the 
many varying individuals can survive ; that such variations 
as happen to lie in a direction which fits the organism to its 
environment will help that organism to survive in com- 
petition with those having a less favorable direction ; that 
the offspring of the surviving organism will inherit the 
variation ; that some will vary in even higher degree ; and 
that thus in time the variation can accumulate to a degree 
which makes its possessor not only a new kind but better 
adapted than its ancestors to those particular conditions. 
Thus Nature acts to select certain characters, and the view 
is known as Natural Selection. Translated into terms of 
the chromosome mechanism, this means that the determiners 
are not stable entities, but exist in a state of unstable equi- 
librium such that they can produce characters in greater or 
lesser degree of intensity. As a matter of fact most of the 
evidence we have accumulated upon this point seems op- 
posed to the idea that the determiners are thus unstable, 
and many investigators deny them all variability. More 
recently, however, some apparently incontrovertible evidence 
has been found which points to an inherent instability of 
the determiners or unit characters, and their modifiability 
by selection ; and the Darwinian conception of evolution 
by selection of such variations will probably prove correct 
in the end. 

A modification of Darwin's explanation of the method of 
evolution is that of De Vries, a Hollander still actively work- 
ing. He maintains, on the basis of observational and ex- 



Ch. VI, 9] METHODS OF PLANT BREEDING 317 

perimental evidence, that some new kinds or species of or- 
ganisms originate not slowly and gradually from other kinds, 
but suddenly, — even so suddenly as in one step from parent 
to offspring. Such new steps are supposed to be not frequent, 
but occasional, long periods of stability alternating with 
short periods of change. Upon forms thus originating 
natural. selection operates to preserve the best fitted kinds. 
The species which thus originate, called often elementary 
SPECIES, differ really, though only slightly, from those which 
give rise to them ; and several mutational steps are needed 
to make such markedly different species (Linn^ean species), 
as the older naturalists associated with that word. This 
view is known as that of Mutation. Translated into terms 
of the chromosome mechanism, it means that the determiners, 
after long periods of transmission in stable form, suddenly 
alter, apparently not by the interpolation of new ones so 
much as by spontaneous sudden change in the old. But 
the evidence on this matter is still in debate. 

9. The Methods used by Man in Breeding Better 

Plants 

Everybody knows that our most valued varieties of farm 
and garden plants — our grains, fruits, vegetables, and 
flowers — do not occur wild, but have been developed by 
man from inferior wild kinds. Our principal grains. Wheat 
and Corn, have been so far improved from their wild an- 
cestors that we know only doubtfully what those were. 
Our best known fruits, Apples, Pears, and Oranges, are 
incomparably superior to the original kinds in size, flavor, 
and other qualities we value. Among vegetables, the 
Cabbage, Cauliflower"; Brussels Sprouts, and others, most 
diverse in aspect, are all known to have been derived in 
gardens from a very simple little strand plant of western 
Europe. In flowers, a great many of our garden favorites 
have been improved from their wild states to a degree 
which would render the relationship unsuspected were it 



318 



A TEXTBOOK OF BOTANY 



[Ch. VI, 9 



not for our historical records. Most remarkable of all, and 
perhaps the acme of man's developmental accomplishments, 
is the Chrysanthemum, in which, from two little simple 
wild plants, smaller than our common field Daisies, have 
been developed all of the great variety of distinct types, and 
all of the superb individual specimen plants seen in our 
horticultural exhibitions, culminating in single plants over 
sixteen feet across and bearing fifteen hundred blossoms, and 
in single blooms over twenty inches in diameter. We 
consider now the methods by which man has achieved 
these results. 

New varieties originate under cultivation, but not as a 
direct result thereof. High cultivation can supply the con- 
ditions for the best development of individual plants or a 
given crop, but the improvement is not hereditary, and 
therefore does not yield new kinds, which we acquire in only 
three ways, — by selection of variations, preservation 
OF SPORTS, and hybridization. 

1. Selection of variations. Both experience and experi- 
ment attest that plants of the same variety growing side by 
side, whether wild or in gardens, present many differences, 
or variations, from one another ; further, that some of these 
variations are hereditary, though many are not ; and still 
further, that by persistent selection generation after gen- 
eration of the plants displaying a given variation {e.g. size 
in a grain, red color in a flower), and the use of their seeds 
in growing the next crop, there results in time a variety in 
which the given feature is far more prominent and prevalent 
than in the original form, and moreover comes true to seed. 
It is true that much of such selection now practiced upon 
highly developed varieties of plants, whether grains or flowers, 
appears to consist simply in the assembling together of the 
plants which already possess the variation in high degree, 
and is not accompanied by any actual intensification 
thereof. In other words, selection may effect the isolation 
rather than the development of a variety. But an intensifi- 



Ch. VI, 9] METHODS OF PLANT BREEDING 319 

cation of variations must sometime and somehow occur, else 
we could never have obtained our multiform and multi- 
chrome Chrysanthemums from their comparatively uniform 
and simple wild ancestors ; and the variation once intensified, 
by whatever method, could be isolated to a variety by se- 
lection. This method of improvement by selection is slow, 
but is favored by use of great numbers of plants, and by the 
fact that plants vary more rapidly and extremely under 
cultivation than in the wild state. In this indirect way, 
indeed, cultivation does promote the development of new 
varieties. 

2. The preservation of sports. Occasionally some one bud 
on a plant will produce a branch having leaves, flowers, or 
fruits strikingly different from those on the rest of the plant, 
such a feature being called a sport. If, now, that particular 
branch be propagated by cuttings or by grafting, the new 
feature holds true; and thus the plants which contain 
it can be multiplied indefinitely. The Red, or Copper, 
Beeches, familiar lawn trees, originated in a single red- 
leaved branch on an ordinary Green Beech, and have 
since been propagated and multiplied by grafting. The 
Navel Orange, which is seedless, and further distinguished 
by the small accessory Orange within its upper end 
(page 201), originated in a sport branch upon an ordinary 
Orange tree, and has been preserved and spread by bud- 
ding (a form of grafting). Indeed, most highly developed 
fruits have originated thus ; somebody has found them as 
sports upon more ordinary kinds, and preserved them by 
grafting. If the sporting branch cannot be propagated by 
cuttings or by grafting, the sport cannot be preserved at 
all, for bud sports are not reproduced by their seeds, which 
produce only the original form. Sometimes, however, 
SEED SPORTS appear, in which case the sports come true to 
seed and can thus be propagated, as in case of some fruit 
trees and a few garden herbs. 

The mode and causes of origin of sports are unknown. 



320 A TEXTBOOK OF BOTANY [Ch. VI, 9 

They occur in all degrees, from barely perceptible to very 
striking, from useless to valuable, and from ugly to attrac- 
tive, — only those which appeal in some way to man's in- 
terests being noted and preserved. They are clearly in the 
nature of extreme variations, which merge over also to mon- 
strosities (page 205) ; and, whatever the case with bud sports, 
no distinction is apparent between seed sports and those 
mutations or hereditary variations upon which selection 
works. 

3. Hybridization. When two parents belong to different 
varieties or species, their offspring are called hybrids, and 
the process of making such crosses is called hybridization. 
Only closely related kinds of plants or of animals can be 
hybridized, presumably because the process requires a cer- 
tain degree of chemical similarity in the complicated pro- 
toplasm. To make the cross in plants, the pollen from a 
flower of one parent must of course be transferred to a 
stigma of a flower of the other parent, which process is usually 
effected by aid of a fine brush. It is also indispensable to 
prevent the access to that stigma of any other pollen, in- 
cluding the plant's own. This end is accomplished by re- 
moving the anthers before they are ripe and covering the 
flower completely with a gauze bag which excludes cross- 
pollinating insects. 

Hybrids show four distinctive characteristics important 
in plant improvement. First, hybrids are apt to be larger 
and finer plants than their parents, although, owing to the 
operation of Mendelian segregation, this feature is not pre- 
served in the next generation. It may, however, be kept 
by use of cuttings or grafting. Second, entirely new fea- 
tures, not apparent in either parental line, may appear, 
seemingly not simply as a result of mixing two ancestral 
strains, but through a kind of sporting induced by the dis- 
turbance incident to the wide crossing. Third, a given 
undesirable character may be bred completely out of a race 
and replaced by a better, on the principle of Mendelian 



Ch. VI, 9] METHODS OF PLANT BREEDING 321 

segregation (page 312), which apphes in full force to hybrids, 
where indeed it was discovered. Fourth, two, or more, de- 
sirable qualities belonging to different varieties may be 
brought together and permanently combined in a single 
variety. Theoretically this is the highest utility of hy- 
bridization, and its practice the highest form of plant breed- 
ing. 

Hybridization is, however, by no means so simple in 
practice as in principle. It is often very difficult to accom- 
plish mechanically; many plants which one desires to 
hybridize fail to set seed with one another's pollen ; new 
features are as likely to be useless as desirable ; hybrids 
designed to combine certain good qualities are as likely to 
combine others which are bad ; the reproductive power of 
hybrids is usually poor ; and many other difficulties make 
hybridization a slow and difficult method of effecting de- 
sired improvements in plants. Nevertheless, in the hands 
of skilled breeders, it is the most important of the three 
methods of plant improvement, and is actually yielding most 
valuable results, especially in the breeding of grains. 

It was earlier said that cultivation, though it makes better 
plants and crops, does not produce new varieties. Indi- 
rectly, however, it helps to that end ; for under cultivation 
plants vary and sport far more profusely and widely than 
when wild, — apparently because of their better nutritive 
conditions, in conjunction with the stimulative effect of new 
surroundings, and perhaps the removal of old restraints. 
Further, it is possible, by devices of cultivation, to intensify 
the rapidity and degree of variation, though not to direct its 
character ; and skilled breeders can thus ''break the type," in 
their phrase, as a foundation for new varieties. It is also 
of course true that the greater the number of plants grown, 
the greater the chance for the appearance of new and de- 
sirable variations ; and this method of growing plants in vast 
quantities is one of the ''secrets of success" of the best 
known of present-day plant breeders, Luther Burbank. 



322 A TEXTBOOK OF BOTANY [Ch. VI, 10 

By a combination of the methods here described, our 
cultivated plants have been developed irom their wild 
ancestors. Obviously the process is a kind of evolution, in 
which man's needs or fancies play the part of the selecting 
and preserving agency. The methods do not include any 
way of originating any desired feature ; all we can do is to 
select, preserve, and intensify such features as nature 
offers. 

In earlier times most, or all, of man's improvements in 
plants were without plan or forethought, his selection being 
made upon features which pleased him, or seemed profitable, 
at the moment; and it is only because in general he has 
continued to be pleased by the same things that our culti- 
vated plants have been brought to their present high de- 
velopment. In modern times, however, much of the im- 
provement is accomplished by expert workers who proceed 
with deliberate forethought and a definite aim in mind. 
This is typical plant breeding, to which we may confidently 
look for great triumphs in the future. 

10. The Morphology of Flowers 

Although the flower is physiologically a distinct organ of 
the plant, having its own primary function of effecting fer- 
tilization, its structure shows obvious morphological relation 
to leaves and stem. 

The sepals of flowers are commonly green, and so leaf-like 
in origin and anatomy as to permit no doubt that they, at 
least, are morphologically identical with leaves. Besides, 
the most perfect gradations occur from sepals through bracts 
to the green leaves of the stem {e.g. Calycanthus). Petals, 
also, despite their difference in color, have a perfectly leaf- 
like development and anatomy, with an occasional complete 
gradation to sepals {e.g. Cactus flowers) ; so that they too 
are morphologically leaves. As to the stamens, the fila- 
ments correspond to leaves in all the morphological test 
points, including a transition to petals {e.g. in Water-lilies), 



Ch. VI, 10] MORPHOLOGY OF FLOWERS 



323 



so that they hkewise are leaves, of a hnear or needle-like 
sort. The anther, however, answers to nothing in a leaf, 
and we hold it in reserve for a moment. In the pistil 
each carpel has the leaf origin and anatomy, its development 
being such that it infolds with the upper surface inward 
(Fig. 221). Where the edges of the infolded leaves grow to- 
gether, the tissues are enlarged, forming placentce (Fig. 222), 
upon which stand the ovules, while the 
tips of these leaves become prolonged and 
modified to styles and stigmas. The ovules, 
however, do not answer to anything in a 
leaf, and we reserve them, Hke the anthers, 
for the present. The receptacle is very 
clearly a stem, enlarged at the tip to 
bear the other floral parts. Sepals, petals, 
stamens, and carpels all stand in whorls, 
which, as with whorls of green leaves on 
the stem, regularly alternate (page 140, and 
Fig. 94), while other relations of phyllotaxy 
occur in these parts. Furthermore, as with 
ordinary leaves and stems, flowers originate 
in buds, which are either terminal or axillary. 
Thus the typical simple flower consists mor- 
phologically of a branch, of limited, or 
determinate, growth, containing whorls of 
modified leaves borne close together at 
the end of a stem, and surrounding two en- 
tirely different kinds of structures, anthers 
and ovules. 

We turn now to examine the morphological nature of 
anthers and ovules, which involves the relations of flowers to 
the reproductive structures of the lower kinds of plants. It 
happens, unfortunately, that not all of the stages which 
must have existed in the evolution of the flower are now 
representee^ in existent plants; but, as will be shown in 
detail in Part II of this book, enough of the stages survive 




Fig. 221. — Dia- 
grammatic repre- 
sentation of the 
mode of union of 
three carpellary 
leaves into a one- 
celled ovary. The 
united edges form 
the placentae, on 
which the ovules 
are borne. (After 
Gray.) 



324 A TEXTBOOK OF BOTANY [Ch. VI, 10 

to indicate the general course of that evolution. Thus we 
can trace the anthers and pollen grains back without any 
serious break to sporangia (or spore ca^es) and spores 
(the kind called microsporangia and microspores) of the 
highest flowerless plants, each anther being a composite 
microsporangium and each pollen grain a microspore. 
We can trace the ovules back in the same way to mega- 
sporangia and MEGASPORES (Fig. 223), each nucellus being 
a megasporangium, and the embryo sac a megaspore, while 
the integuments are a special new outgrowth from the 
stalk of the sporangium. We can, however, trace these 
parts still farther back to an origin in a single kind of sporan- 




FiG. 222. — Diagrams to illustrate, in cross section, the various ways in 
which carpels, here five in number, unite to form compound pistils and 
placentae. 

First, carpels all separate; second, united like Fig. 221, giving parietal 
placentae ; third, infolded to the center, like the first but grown together, 
giving central placentae; fourth, like the third, but with the partitions 
wanting, giving free central placenta. 

gium and spores, such as we find in the Ferns, where they 
occur in the brown sori, or ''fruit dots," on the backs of the 
fronds (Fig. 224), and we can even trace them, if we choose, 
back into the Algse. Thus we see that pollen grains with 
the anthers, and embryo sacs with the ovules, are mor- 
phologically equivalent to the spores and spore cases of the 
lower plants, and are therefore far older than the other 
parts of the flower. Hence a flower consists morpholog- 
ically of stem, leaves, and sporangia with their spores. Or, 
since the spores are the more important as well as the older 
parts, we may say that morphologically a flower consists of 
spores together with stem and leaves speciahzed to aid in 
their reproductive function. 



Ch. VI, 10] MORPHOLOGY OF FLOWERS 



325 



This identification of pollen and 
ovules with the spores of the lower 
plants at once throws light on two other 
features of floral structure. First, the 
megasporangia and microsporangia of 
the flowerless plants occur in close asso- 
ciation with, or upon, certain leaves, 
somewhat modified accordingly, called 
SPOROPHYLLS (Fig. 223) ; and it seems 
clear that stamens and pistils are the 
lineal descendants of the sporophylls. 
As to petals and sepals, it is not yet 
certain whether they represent ancient 
sporophylls which have lost their spo- 
rangia, or green leaves independently 
specialized, though the latter seems 
most probable. Second, the pollen 
grains and embryo sacs (the ancient 
spores) are not themselves the sex cells, 
but develop the sperm cells and egg 
cells through intermediation of some 
cell divisions which have no apparent 
meaning under existent conditions (Figs. 
188, 190, and full account in Part II). 
Now in the lower plants the spores 
are not sex cells either, but they pro- ing strobiius of Seiagi- 

- . nella moequifoha, a 

duce special and oiten elaborate struc- pteridophyte ; x 12. 
tures (including the prothallus stage ^^ *^e left, micro- 

r xi_ T^ xi_ ^1- n i? XT- T • sporangia containing 

of the Ferns, the thallus of the Liver- several microspores ; on 

worts, and the whole body of the the right megasporangia 

,, s 1 • 1 .1 n containing four mega- 

MoSSes), upon which the sex cells are spores. The sporangia 

developed ; and it is the reduced pro- ^tand upon sporophylls. 
thallus, or equivalent, of the lower 
plants which persists as the seemingly meaningless cell divi- 
sions within the pollen grain and embryo sac. Thus while 
ovule and embryo sac, with anther and pollen grain, are parts 




Fig. 223. — The fruit- 



326 



A TEXTBOOK OF BOTANY [Ch. VI, 10 




of the flower, the prothallial cells 
of both embryo sac and pollen 
grain, together with egg cell and 
sperm cell belong to a new gen- 
eration. 

These morphological matters 
are certainly complicated and 
difficult at first to grasp in detail. 
They can be made clearer, how- 
ever, by aid of a table or dia- 
gram which will exhibit their 
relations in light of their evolu- 
tionary origin, and of the con- 
nections of the reproductive with 

the nutritive parts ; and such a diagram is presented on 

the opposite page. 

We have now traced the flower back to its morphological 



I'iG. 224. — Sorus of a fern, 
in cross section, showing the 
stalked sporangia containing 
spores ; magnified. From these 
spores there is an unbroken 
series to the embryo sacs and 
pollen grains of flowers. (From 
F. Darwin.) 





K°ffi°;i [{Xyo}] ;uo)j! 



Fig, 225. — Plans, or diagrams, of typical flowers, to illustrate presence 
and absence of the whorls. 

They represent cross sections supposed to be made through the widest 
parts of sepals, petals, stamens, and pistil. Above, the first is a complete 
flower (Staphylea), and the second is apetalous (Beet). Below, the first is 
asepalous and apetalous (Saururus) , the next is staminate only (Willow) , and 
the last is pistillate only (Willow). 




ssxxRdoiTNsaas saiAHdooiaatd sisAHdoga saiZHdoiiYHX 

327 



328 



A TEXTBOOK OF BOTANY [Ch. VI, 10 



foundation, but have still to trace it upward through a 
remarkable morphological elaboration. 

Typically the flower has sepals, petals, stamens, and carpels 
(Fig. 225), but these may be absent in various degrees, making 
the flowers apetalous, asepalous, pistillate, or staminate, all 
of which terms are self-explanatory. 

Typically all of the whorls have the same number of parts, 
as in the phyllotaxy of leaf whorls on the stem (page 140, 
Fig. 94). That number is oftenest five (Fig. 226), no doubt 
because of the predominance of the | system of phyllotaxy 
(page 141) ; next most often it is three, connected with the 
J system ; while less often it is four, presumably connected 





Fig. 226. — Diagrams of typical flowers, to illustrate the principal 
numerical plans. Constructed as in Fig. 225. 
5-plan, Oxalis ; 4-plan, Fuchsia ; 3-plan, Lily. 



with the i system; and these are the only numbers which 
prevail through flowers. This relation to phyllotaxy, by 
the way, shows how purely structural and httle adaptationai 
is the numerical feature of floral structure. Any of the four 
whorls may deviate from the number characteristic of the 
flower. Thus Poppies have but two sepals. Monkshood has 
but two petals. Orchids have but one or two stamens, and 
Peas have but one carpel. As to the stamens, they are some- 
times fewer, but often are more numerous than the typical 
number, especially in simple flowers pollinated by many 
insects, such as Roses and Buttercups. The carpels, on the 
contrary, rarely exceed the typical number (though they do 
so in both of the plants last mentioned), but oftener than 



Ch. VI, 10] MORPHOLOGY OF FLOWERS 



329 



not are less than the prevalent number, being commonly 
three in a 5-part flower, or even only one, as prevails through 
the great Pulse family (Fig. 227). In general a diminution 
in number accompanies increasing efficiency in function, and 
marks a higher grade in evolution. Thus the Composite 
family (that of the Sunflower and Chrysanthemum), the 
largest plant family, and the one which stands highest of 
ah in plant evolution, has five sepals (when any), five petals, 
five stamens, and 
one carpel. 

As the floral leaves, 
especially the sepals 
and petals, develop 
and broaden in the 
bud, their edges be- 
come variously dis- 
posed with respect 
to one another. In 
some flowers these 
parts have their edges 
exactly matching to- 
gether without any 
overlapping, as in the 
sepals of Fuchsia 
(Fig. 226), an ar- 
rangement called 
VALVATE. In others 
the edges regularly overlap spiralwise, as in the petals of 
Fuchsia, an arrangement called convolute. Oftenest they 
overlap in such manner that some parts have both edges 
under, some both over, and some both ways, an arrangement 
called IMBRICATE (Primrose in Fig. 227). These arrange- 
ments, called collectively estivation, often persist in the 
open flowers, though sometimes so hghtly as to be easily dis- 
arranged by a touch or the wind. They are apparently due 
to a combination of phyllotactic and developmental factors. 




Fig. 227. — Diagrams of typical flowers, to 
illustrate deviations from numerical syrametry. 
Constructed as in Figs. 225, 226. Above, 
Stellaria and Cassia ; below, a Composite 
(Helenium) and Primrose'. 



330 A TEXTBOOK OF BOTANY [Ch. VI, 10 

Typically, and usually, the floral whorls alternate, as in 
the case of leaves on the stem (page 140). Most of the 
exceptions are only apparent, as in the Lily family (Fig. 226), 
where a whorl of six stamens seems to stand opposite a 
whorl of six petals or sepals (e.g. Lily of the Valley) ; but 
in reality whorls of sepals and petals, here alike, and two 
whorls of stamens regularly alternate. In case of the 
Primrose, where five stamens stand opposite five petals 
(Fig. 227), it is Hkely that another set of five stamens, which 
would make the alternation perfect, has vanished in the 
course of evolution. Indeed, two whorls of stamens are 
more frequent, and perhaps more 'H3rpicar' than one. 
The usual lesser number of carpels, of course, destroys the 
alternation in their case. 

Typically the sepals, petals, stamens, and carpels all 
stand separate and distinct upon the receptacle, precisely 
as do leaves on the stem ; but sometimes each whorl forms a 
single structure. Thus the calyx, as earUer noted (page 270), 
is often one structure at base, and even to near its top, while 
sometimes it forms a tube with only small teeth on its free 
margin, e.g. Phlox. It was formerly supposed that such a 
calyx is formed by a union of the lower parts of the sepals, 
the tips alone remaining free, on which account it was called 
GAMOSEPALOUS (united sepals) in distinction from polysep- 
ALOUS applied to the separate condition. This view, how- 
ever, finds no support in the development of the indi- 
vidual flower, where no such union of parts takes place; 
for, in fact, the sepals originate and grow separately for a 
time, and then are lifted by the growth of a continuous ring 
of leaf -like tissue, which gradually elongates to the tubular 
part of the calyx. It is possible that in course of their 
evolution the sepals have become united, as the older view 
held; but it is equally possible, and much more in accord 
with the method of their present development, that only the 
free tips represent the original separate leaves, while the 
tubular part is a new development, just as we know the 




Fig. 228. — Diagrams of typical flowers in vertical section, showing the 
various relations of calyx, corolla, stamens, and carpels, as interpreted by 
their development from the buds. 

Receptacle is dotted ; floral tube is lined lengthwise ; carpels are lined 
crosswise. The parts in broken line do not fall in the median plane in a 3- 
plan flower. 

Upper based on Scilla ; next on Hyacinth ; next on Snowdrop ; lower on 
Narcissus. 

331 



332 



A TEXTBOOK OF BOTANY [Ch. VI, 10 



external tube, or corona, of the Daffodil to be. Precisely 
the same is true of the gamopetalous corolla, and also of the 
monadelphous stamens, although in cases where the stamens 
are united, as in the Compositse, these anthers do actually 
grow together although they originate separately. As to 
the carpels, where two or more unite into a single pistil, 




Fig. 229. — Fuchsia speciosa, showing the raceme of morphologically 
specialized flowers, with inferior ovary, and both petals and stamens raised 
on the calyx tube. (From Bailey.) 



the case is quite clear, for they always originate separately 
in the bud, and later actually grow together as they develop. 
The mode of fusion of the carpels determines the place of 
the placentae and the number of compartments (unfor- 
tunately called cells) in the ovary. Thus in the Pulse family, 
illustrated by the familiar green Pea, only one carpel is 
concerned, and it infolds with a single parietal placenta 
(Fig. 227). When two or more carpels unite to one pistil, 



Ch. VI, 10] MORPHOLOGY OF FLOWERS 



333 



they may grow together in any of the ways shown in 
Figure 222, producing parietal, central, or free central pla- 
centae, with one or several compartments. 

Typically each of the four whorls stands directly on the 
receptacle independently of the other three ; but remarkable 
interrelations of the whorls also 
occur in various flowers, as repre- 
sented diagrammatic ally in Fig- 
ure 228. In some cases the 
calyx and corolla together form 
one structure, called perianth, 
upon which stand the stamens, 
as in the Hyacinth, while vari- 
ous other combinations occur. 
Formerly such cases were inter- 
preted on the supposition that 
the different whorls were united, 
or adnate, to one another from 
the receptacle upward; but 
here also the development of 
the flower favors another inter- 
pretation, viz. that the tube 
which the parts occupy in com- 
mon has developed in precisely 
the same way as the tube of the 

,, , . . Fig. 230. — The Daffodil, Nar- 

COrOlla or calyx, not by a ^^-g^^^ Pseudo-Nardssus, showing 

union of originally free parts, the large corona, an outgrowth 
, . ±^ ' ^ from the sepals and petals. (From 

but as a new growth mter- Bailey.) 
calated between the free struc- 
tures and the receptacle. Especially striking is the con- 
dition of inferior ovary (page 275), where sepals, petals, 
and stamens stand upon its top (third flower. Fig. 228). 
This arrangement was formerly interpreted on the sup- 
position that the calyx (and therefore also the corolla 
and stamens) was united or adnate to the ovary all the 
way up from the receptacle below; but here also the 




334 



A TEXTBOOK OF BOTANY [Ch. VI, 10 



development of the flower favors a different interpretation, 
viz. that the receptacle grows up in cup-shaped form, carry- 
ing upon its top the four whorls, of which the carpels come 
simply to close in the roof of the ovary, as represented in 
the lower diagrams (Fig. 228 ). In case of the Apple, the up- 
growing receptacle appears to have inclosed the set of carpels, 
represented by the core. Yet these distinctions of floral 

parts have in reality no great weight, 
since as the flower becomes special- 
ized the former sharp distinction 
between stem and leaves, and even 
that between receptacle and floral 
tube, tends to disappear. This 
consolidation of the parts of the 
flower goes still farther in cases 
like Fuchsia, where the floral tube 
stands upon the ovary, and upon 
the tube stand sepals, petals, and 
stamens (Fig. 229) ; and it reaches 
perhaps its perfection in the Orchids 
where even the stamens and pistil 
form one mass. 

Typically the sepals, petals, sta- 
mens, and carpels follow the method 
of leaves in their development, and, 
like leaves, branch readily in their 
own plane, but rarely out of it. Yet 
the floral parts do at times produce 
special outgrowths from their faces, 
as in case of some nectaries, the scales in the throats of 
some Pinks, and the remarkable ''crown of thorns" in 
the Passion flower. Somewhat similar in origin is the corona 
of the Narcissus, a structure which in the Daffodil (Fig. 230) 
surpasses in size and prominence even the regular floral 
tube itself. 

In such features as these outgrowths, and in many of the 




Fig. 231. — Cymes, com- 
pound, of the Wild Geranium 
(From Bailey.) 



Ch. VI, 11] MORPHOLOGY OF CLUSTERS 



335 



other facts of progressive consolidation and specialization 
of parts above described, we see that the flower is by no 
means closely bound by its former leaf and stem nature, but 
has acquired in large measure its own morphological inde- 
pendence. It is therefore in effect a morphological member 
as well as a physiological organ of the plant. 



11. The Morphology and Ecology of Flower Clusters 

The conspicuousness of flowers, 
especially of the smaller kinds, is 
greatly augmented by their aggre- 
gation into clusters. There is 
more, however, in the subject than 
this, for clusters often exhibit a 
specific individuality, with distinc- 
tive new characters of their own. 
In wind-pollinated kinds, where 
showiness has no functional value, 
the clusters have apparently no 
more than a structural significance, 
as a convenience of development. 

Each flower originates in a bud, 
representing morphologically a 
spore-bearing determinate branch 
(page 323) ; and flower buds, like 
leaf buds, are usually either termi- 
nal or axillary. Now every possible 
gradation is found between a con- 
dition in which solitary flowers are 
scattered along stems in the axils 
of green leaves and that in which 
numerous flowers are massed densely 
together with the leaves reduced to 
insignificant bracts or wanting al- Fig. 232. — Eremums 

J ji -nrri ,1 T, himalaicus, showing a rac- 

together. Where the solitary con- emose spike of flowers. 
dition ends and a cluster begins is (From Bailey.) 




336 



A TEXTBOOK OF BOTANY [Ch. VI, 11 




Fig. 233. — Button Bush, Cephalanthus 
occidentalis, showing the head of flowers. 
(From Bailey.) 



largely an arbitrary mat- 
ter, determined in practice 
by whether leaves or 
flowers are more promi- 
nent in the mass. In 
many, perhaps most, 
cases, however, there is no 
difficulty in distinguishing 
a cluster, because it ex- 
hibits a sharp transition to 
the leafy stem ; and this 
distinctness constitutes 
the first step in the indi- 
viduality of the cluster. 

The simplest clusters 
are those in which a con- 
tinuously growing stem 
produces a flower in the axil of each reduced leaf, the older blos- 
soms being thus below and the younger above, — and often the 
lower become fruits while the 
upper are still buds. Such a 
cluster, commonest of all 
kinds, is caUed a raceme 
(^Fig. 229). In marked mor- 
phological contrast therewith 
is the CYME (Fig. 231), in 
which a terminal flower closes 
the growth of the stem, and 
the new flowers appear from 
buds progressively lower 
down. The two types, called 
respectively indeterminate 
and determinate, corre- 
spond exactly with the defi- 
nite and indefinite annual 
growth of stems, earlier described (page 138). 




Fig. 234. — Corymb of Cherry. 
(From Figurier.) 



Ch. VI, 11] MORPHOLOGY OF CLUSTERS 



337 



Both racemes and cymes often become compound by the 
branching of the main flower stalks, and the two types occur 
intermingled in the more complicated clusters, such as the 
pyramidal thyrsus of the Lilac and Horse-chestnut and the 
much looser panicle of the Meadow Rue, and most of the 
loose-topped Grasses. In the other direction, the clusters 
become very compact. Thus racemes sometimes have so 
many flowers on such short 
stalks as to form collectively 
a SPIKE (Fig. 232), as familiar 
in Mullein, while if bracts in 
a spike are more prominent 
than petals, as so commonly 
occurs in wind-pollinated 
trees, we have a catkin, 
familiar in Birches (Fig. 197) 
and ' ' pussy willows . ' ' If the 
main stem remains short, 
bringing the flowers all close 
together, the cluster is a head, 
as familiar in Clover and 
Button Bush (Fig. 233). 

The clusters thus far noted 
are little more than aggrega- 
tions of similar flowers, but 

1 . 1 ■, 1 1 1 1 • 1 Agapanthus umbellatus. 

more highly developed kinds Bailey.) 
show a marked approach to 

the aspect of single large flowers. The tendency is first 
manifest in the production of flat-topped clusters. Thus, 
if the main stem and the stalks of the lower flowers of a 
raceme all elongate at about the same rate, there results 
a flat-topped corymb (Fig. 234). When, further, the 
main stem remains still shorter, or undeveloped, and the 
flower stalks have all about equal lengths, there results a 
characteristic umbel (Fig. 235), a very common form of 
cluster, and one which prevails through, and has given name 




Fig. 235. 



A typical umbel, of 
(From 



338 



A TEXTBOOK OF BOTANY [Ch. VI, 11 



STERILE 
^ FLORETS 



MALE 
i|FLORETS 



to, a large family of plants, the Umbelliferse. Both 
corymbs and umbels also become branched or compounded. 
Still more advanced in evolutionary rank are those clusters 
in which there is found a division of labor 
with respect to the functions of reproduction 
and conspicuousness. In some clusters the 
conspicuousness which shows the flower to 
insects is given by bracts greatly developed, 
as with the Calla and Jack-in-the-pulpit, 
where the single showy bract or spathe acts 
functionally like a corolla, leaving only the 
function of pollination to the little incon- 
spicuous flowers arranged on a fleshy spike 
called a spadix (Fig. 236). Bracts also form 
the showy parts of the flat-topped clusters 
of comparatively inconspicuous flowers in 
Poinsettia and Flowering Dogwood. Still 
more highly developed are those clusters in 
which this division of function occurs be- 
tween the flowers themselves. Thus, in the 
wild Hydrangea and its relatives, the inner 
flowers of the flat-topped compound cyme 
remain inconspicuous, and the showiness of 
the cluster is due to the petals of the outer- 
most flowers which have developed very 
greatly (Fig. 237), losing entirely in the 
process their reproductive parts. It is these 
outer NEUTRAL flowers which have been de- 
veloped in cultivation to form the fine great 
showy pyramidal clusters (thyrsi) of our lawn 
Hydrangeas. This arrangement reaches its 
highest development in the family Com- 
positae, where, in forms like the Sunflower, 
the outer row of the flowers (the so-called ray flowers) in 
the dense, flat-topped cluster develop greatly their corollas 
which make the whole showy parts of the head, but lose their 



BASE 

OF 

SPATHE 



Fig. 236. — 
The spadix, with 
flowers, of an 
Arum ; the large 
showy spathe is 
removed. (From 
Cavers.) 



Ch. VI, 11] MORPHOLOGY OF CLUSTERS 



339 



stamens and often also their pistils in so doing ; while simul- 
taneously all of the interior flowers (the disk flowers) 
remain comparatively inconspicuous and devoted entirely 
to pollination. So far, indeed, does the resemblance to 




Fig. 237. — Flower cluster of Hydrangea Bretschneideri, 
corymb with showy neutral flowers. 

Lower left ; certain details of the fruit. (From Bailey.) 



a compound 



large single flowers proceed that even a calyx-like structure 
(called involucre) is developed from bracts, these collective 
features giving the clusters so much the aspect of single 
flowers that they are popularly thought to be so. The resem- 
blance, indeed, appeals even to insects, which visit and 



340 A TEXTBOOK OF BOTANY [Ch. VI, 12 ' 

pollinate the clusters in precisely the same way as they do i 
single flowers. These heads in the Compositae represent j 
the highest evolutionary development of clusters. j 

\ 

12. Special Forms, Abnormalities, and Monstrosities ! 

OF Flowers '\ 

Although leaves, stems, and roots often perform functions 
and have forms very different from those which are pri- 
mary and typical in those organs, flowers have hardly any 
additional or substitute functions, doubtless because of [ 
their high degree of specialization to their primary function, i 
On the other hand, flowers far surpass all other organs in * 
the abundance of their abnormalities and monstrosities, ! 
presumably because their much greater complication of : 
structure allows more opportunity therefor. i 

Abnormal or monstrous flowers, those which deviate in some j 
unusual or eccentric way from the conditions usual in that j 
kind, are apt to occur in any bed, especially in gardens, — 
for they are more frequent under cultivation. , 

The monstrosities occur in all possible parts. Sepals are i 
found, either singly or the whole whorl, entirely leaf-like in 
size and appearance, even to complete compounding in [ 
some Roses. Also they occur so petal-like in color and form ! 
as to resemble a seemingly two-storied flower, as in ''Hose in ! 
hose" Primroses. Petals act in many strange ways, even ; 
turning leaf-green in some monstrous Roses. They are j 
especially prone to multiply much in number, giving us 
double flowers, of which a great many kinds can be propagated, 
and occur in our gardens. Stamens are sometimes completely 
petal-like ; sometimes bear ovules in their anthers instead 
of pollen ; sometimes are completely replaced by carpels. 
Carpels often fail to unite their edges, thus leaving the ovary 
open ; and they become in various degrees leaf-like. Some- 
times the ovary contains anthers with pollen instead of 
ovules, and sometimes the ovules are replaced by tiny 
green leaves. The receptacle also acts diversely, its most 



Ch. VI, 12] MONSTROSITIES OF FLOWERS 341 

frequent abnormality consisting in a continued growth right 
up through the center of the flower, above which it produces 
a second flower, or else a leafy branch, as already described 
in connection with stems (page 201). Sometimes two or 
more of these abnormalities are combined in a single flower, 
in which case we have a genuine, and often an extreme, 
monstrosity (Fig. 150). One or more of the whorls may be 
absent though normally present, or present when normally 
wanting ; and any or all may become altered in color, 
multiplied in number, or converted entirely into a bunch of 
green leaves. Regular flowers become diversely irregular, 
and irregular kinds perfectly regular. Also flowers, especially 
their pistils, become malformed to galls under insect stimula- 
tion (page 203). It is surprising how many and diverse are 
the abnormalities which appear when one's attention is 
directed to these matters, and how many are described 
and pictured in the special works devoted to the subject. Of 
the latter the most famous and instructive is the classic 
" Vegetable Teratology " by Masters, which the student will 
do well to examine. 

Not only structural, but physiological abnormalities occur, 
as for example in cases where the " resting-period " (page 378) 
is wanting, and the flower opens in autumn instead of the 
next spring, as happens with exceptional Strawberry blossoms 
and flowers of shrubs. Of course such flowers are destroyed 
by frost without chance to form seed. Sometimes the ab- 
normality, especially in extreme monstrosities, occurs only 
in a single flower, in which case it is usually not hereditary 
and cannot be propagated, just as with fluctuating varia- 
tions (page 314). But sometimes all of the flowers on one 
branch or one plant exhibit the feature, in which case it 
can usually be propagated like a sport, which indeed it 
really is, — both bud sports and seed sports of this kind 
occurring. Hence we have in our collections the permanent 
strain of the "Hose in hose" Primrose; in our greenhouses 
we have a green Rose propagated as a curiosity; and in 



342 A TEXTBOOK OF BOTANY [Ch. VI, 12 

our gardens we have double flowers in an extreme abundance, 
the doubUng in some cases being due to the transformation of 
stamens to petals, and in others to a multiplication of petals. 
Thus it is plain that no line can be drawn between variations 
and abnormalities, sports and monstrosities. 

We should now note somewhat more fully the causes of 
monstrosities, as to which we have little exact knowledge, 
though some good circumstantial clews. It was once be- 
lieved that they are mostly reversions to a simpler ancestral 
condition, but further knowledge has shown that they are 
usually reversions to a simpler structural condition. They 
are chiefly due to disturbance in the growth control mecha- 
nism. The development of any organism and its parts 
depends upon three sets of factors : First, there is the supply 
of matter and energy contributed by the metabolism of the 
plant, and as these are supplied to every living cell, all 
parts have thus the power and the impulse to grow without 
dependence upon the others. Second, there is the guidance 
of the development of the particular parts, exercised in 
some way by the chromosomes through the cytoplasm, and 
partly determined by heredity and partly by responses to 
external stimuli. Third, there is correlation between the 
different parts of the plant such that the power and impulse 
of each part to grow far more than it does is kept in restraint 
and subordinate to the development of the organism as a 
whole, as witness the case of buds, sometimes forty times 
more numerous than are permitted normally to develop 
(page 138). As to the mechanism of this correlation we have 
as yet no idea, though it is clear that the physical path of its 
operation lies through the protoplasm which is continuous 
from cell to cell. Now monstrosities can often be traced to 
a failure in operation of some one of these sets of factors, 
but they seem oftenest to result from a failure in the third, 
caused by mechanical damage to the path of conduction (as 
in case of burls, page 200) or by chemical paralysis through 
action of parasites (Witches'-brooms, page 198). When the 



Ch. VI, 13] ECONOMICS OF FLOWERS 343 

control mechanism becomes inoperative while the growth 
energy is still forcing forward the growth of the part, then 
the part seems to be controlled by whatever structural con- 
dition happens to be strongest at the moment. 

13. Economics, and Treatment in Cultivation, of 
Flowers 

Flowers, unlike the five other primary plant parts, have 
few economic uses, aside from the beauty they give to our 
gardens. That, however, is surely a utility of civilization, 
and besides it maintains great business interests in seed 
firms and nurseries which supply ornamental flowers, trees, and 
shrubs. In a few cases perfumes are extracted from flowers, 
which also supply the nectar elaborated by bees into honey. 
But otherwise their direct uses are insignificant. 

Turning to the cultivation of flowers, we find some features 
of gardening practice dependent on their physiology. 

Since showy flowers are cross-pollinated by insects, those 
who grow seeds or fruits for market find it well to keep 
Bees, best of cross-pollinators, in their gardens, or even 
their greenhouses, where crops of Tomatoes or Cucumbers 
are forced for early market. It is true the pollination can 
be effected artificially by use of fine brushes, as often done 
for special purposes; but Bees are more economical. In 
another way this relation of insects to flowers affects practical 
interests, for if the blossoming time of our fruit trees. Apples, 
Pears, and others, falls cold and wet, the insects are not active 
and pollination is only partial, which is one cause of poor 
fruit years. 

The reciprocal balance, already described (page 207), 
between vegetation and reproduction, makes it possible for 
gardeners to promote flowering by checking the stem and 
leaf growth, either through withholding fertilizers, by root 
pruning, or by other devices known in the business. Pruning, 
in orchards, has chiefly this use, as earlier noted (page 207). 
These methods, however, have strict Hmitations, and are 



344 A TEXTBOOK OF BOTANY [Ch. VI, 13 

effective only in skilled hands. Theoretically the best 
results would be attained when a plant has been stimulated 
to vigorous vegetative growth until a large reserve of food 
has accumulated, and then is checked in its stem and leaf 
growth. 

Flowers are prone to wilt when cut, even if placed imme- 
diately in water, because they now lack the root pressure 
which helped their supply. Moreover, their evaporation 
current through the cut ducts draws into the latter various 
micro-organisms which here find such congenial conditions 
for growth that they fill the passages and stop the water. 
The devices for preserving the freshness of flowers are ad- 
justed to neutralize these conditions. Thus, everybody knows 
that flowers keep best in cool, moist, shaded places, — be- 
cause evaporation is there checked; and florists keep their 
Roses before sale in refrigerators for this reason. On the 
other hand, a frequent changing of the water, clipping away 
the lower and often discolored ends of the stems, the addi- 
tion of a little salt, dipping the cut ends for a moment in 
hot water, charring the ends in a flame, — all of them devices 
recommended by different people for preserving particular 
kinds of flowers, — have in one way or another the effect of 
antagonizing the organic growths in the ducts, thus keeping 
the passages open. It is said that white flowers last longer 
after cutting than colored kinds, which perhaps is connected 
with the fact that they absorb less sunlight than colored 
kinds, and hence suffer less evaporation from their tissues. 
Florists have still another device, useful in some cases, de- 
pending on the fact that since petals usually fall immediately 
after fertilization, flowers last longer if that is not effected. 
Fertilization can be prevented by removing the anthers 
from all flowers as soon as they open. This is commonly 
practiced with large LiHes. 



CHAPTER VII 
THE MORPHOLOGY AND PHYSIOLOGY OF FRUITS 

1. The Distingtive Characteristics of Fruits 

The word fruit has far wider significance in scientific than 
in popular language, for to the botanist it includes any 
structure which has part in the development of seeds, no 
matter whether edible or not, or what the aspect it presents. 

Most fruits are the ripened ovaries of flowers, from which 
all other parts (excepting of course the receptacle) have 
fallen away, though occasionally some of the other floral 
parts persist, and become incorporated with the ripening 
ovary. There are fruits, however, which have no connection 
with ovaries, as in berries of Yews and cones of Pines, though 
in such cases other structures replace the ovaries in function. 

The ovary, as a rule, withers and falls with the other parts 
of the flower unless pollination occurs ; but after pollination 
the ovary develops to a fruit, the ovule to a seed, and the 
fertihzed egg cell to an embryo. Thus polhnation acts as 
the stimulus to fruit formation, the arrangement being 
obviously advantageous in preventing the waste of good food 
material upon fruit and seed if no embryo is formed to be 
protected and disseminated, — and no embryo is formed 
without fertilization. 

Fruits display well-nigh as great a diversity in their visible 
features as do the other plant organs. They fall rather 
naturally, however, into two great classes, — dry fruits, like 
pods, and fleshy or edible fruits, like berries. 

In size, fruits are almost microscopic in some very small 
plants, and vary thence upward to the great double Coco- 

345 



346 A TEXTBOOK OF BOTANY [Ch. VII, 1 

nut, a foot or two in diameter, and weighing some thirty 
pounds. The largest fleshy fruit is probably the Jack fruit 
or Durian of the tropics, often mentioned by travelers. 

In shape, fruits are diverse as possible, though tending to 
rounded forms like the ovaries from which they are developed. 
Sometimes they answer very closely to the shape and aspect 
of a single seed, to such a degree as to be commonly mis- 
taken therefor. 

In texture, the difference between dry and fleshy fruits 
becomes very manifest. In dry fruits the walls of the ovary 
are parchment-like or woody, as in most pods, or even al- 
most ivory hard, as in some nuts and fruit pits, while in 
fleshy fruits the ovary walls become soft, pulpy, nutritious, 
and palatable, as we, and other animals, know very 
well. 

In color, the two classes are likewise contrasted. The 
dry fruits are mostly brown or gray, like bark, indicating that 
their color has no bearing on their function, and is simply 
that which happens to be natural to ripening woody tissues. 
The fleshy fruits, on the other hand, are mostly bright colored, 
— red, yellow, purple, and sometimes white, — in marked 
contrast to their respective backgrounds. Such colors we 
naturally assume to indicate a functional connection with a 
seeing eye, — an assumption which proves to be true, as a 
later section will indicate. 

The fruits, of botanical terminology, include some struc- 
tures which are popularly rated as vegetables, notably Cu- 
cumbers, Pumpkins, and Squashes. These, however, are 
forms of fleshy fruits, as their whole structure attests. 

Fruits produce seeds in diverse numbers from one to many 
hundreds. Dry fruits which contain several seeds open or 
dehisce to allow their escape, but fleshy fruits, no matter how 
many their seeds, remain closed, the seeds being released 
in other ways which we shall presently consider. 

As in case of other organs, popular terminology is some- 
what uncritical. Thus the ''fruit-dots" of Ferns have no 



Ch. VII, 2] 



MORPHOLOGY OF FRUITS 



347 



I 



connection with fruits; ''Cedar apples" are only a Fungus 
product; and the ''fructification" of Fungi refers only to 
their spore masses. 

2. The Structure and Morphology of Fruits ' 

The structure and morphology of fruits are largely de- 
termined in the ovaries from which they originate, — fruits 
being primarily ovaries further developed and specialized. 
The particular features of the fruit have usually an obvious 
connection with the method of dis- 
semination of the seeds, — the accom- 
plishment of such dissemination 
being commonly a function of the 
fruit. 

The structural features of the 
ovaries — walls, partitions, number 
of compartments and placentae — 
can usually be recognized clearly, 
and in the same relative connections, 
in the fruits, while the dehiscence, 
or opening through which the seeds 
escape, likewise follows as a rule 
some morphological hues of the 
ovary. Deviations in these features, however, often occur, 
and can usually be traced to a connection with the method 
of dissemination. 

The fruit structure is clearest in dry fruits. Thus a typical 
fruit of the simplest sort is represented in the pod of Colum- 
bine (Fig. 238), which is developed from an ovary of one 
carpel, bearing one row of seeds; these are arranged along 
a parietal placenta, formed where the edges of the carpellary 
leaf unite, and the pod in dehiscence simply dis-unites those 
edges. In the Green Pea, however, of precisely the same con- 
struction, the pod dehisces both by disuniting the edges and 
also forming a new spht along the back or midrib of the car- 
pellary leaf. Pods originating in two or more carpels like- 




FiG. 238. — Pods of Col- 
umbine. (From Bailey.) 



348 



A TEXTBOOK OF BOTANY 



[Ch. VII, 2 




Fig. 239. — Pod of £ 
Poppy; Xi-. 

It stands at the sum- 
mit of a long stiff stalk 



wise usually dehisce by disuniting the 
joined edges, though sometimes they 
spUt also down the carpellary midribs. 
Frequently, however, the dehiscence 
follows no morphological line in the 
ovary, but occurs in new and independ- 
ent positions connected with a par- 
ticular method of dissemination. Thus, 
in the capsules of Poppies new openings 
arise around the tops of the fruits and 
in Purslane the capsule sphts right 
across without any regard to morpho- 
logical lines (Fig. 239) ; in some of the 
Mustard family the carpels mostly spht 
away as valves from the placentae, which 
persist for a time as a framework (Fig. 
240) ; and other arrangements also occur, some of which 
prevail throughout families in ways to show that large 
structural and hereditary factors enter along with adapta- 
tion into the construction of fruits. On the basis of their 
aggregate structural fea- 
tures, the dry fruits are 
classified and named as fol- 
licles, LEGUMES, SILICLES, 
etc., these distinctions hav- 
ing importance in connection 
with the taxonomy of plants. 
The only dry fruits which 
do not dehisce at all are 
those which contain but a 
single seed, as typified by the 
little AKENES of the Straw- 
berry and Buttercup, com- 
monly supposed to be seeds Fig. 240.— Honesty, Lw.narm annua, 
{¥\{r 24n Thpvflrpin fart ^^ ^^^^^ ^^^ persistent partitions of 

functionally seeds, both in (From Bailey.) 




Ch. VII, 2] 



MORPHOLOGY OF FRUITS 



349 




Fig. 241. — The 
seed-like fruits 
(akenes) of Butter- 
cup, one in section ; 
X 5. (From Bailey.) 



dissemination and germination, the ovary wall serving simply 
as an additional pit-like coat. A very important form of 
single-seeded indehiscent fruit is the grain (Fig. 242), dis- 
tinguished particularly by the fact that seed 
coat and ovary wall are grown completely 
together, thus making the structure so seed- 
like that only the botanist knows its true 
morphological nature. As its name implies, 
this fruit is characteristic of the grains, — 
Corn, Wheat, Oats, etc. Nuts also are 
commonly one-seeded, though here we meet 
with morphological complications, both as to the original 
number of the ovules and the nature of the shell. 

While , in general the construction of the fruit answers 
closely to that of the ovary, some exceptions occur, indicating 
that the fruit has a certain morphological 
independence of its own. The development 
of new dehiscence lines is one instance 
thereof. The number of compartments, or 
cells, is usually the same in ovary and fruit, 
but sometimes partitions disappear, or new 
ones develop ; while we find also such 
changes as the formation of four little nut- 
lets (prevaihng throughout the Mint family) 
from a two-celled ovary. Not infrequently 
a several-celled ovary produces a one-celled 




ZM\ 



Fig. 



242. 



^^,^:^ -^nd one-seeded fruit, as in most of our com- 

the embryo, R, G, 
endosperm. A, and 
the united seed and 
ovary coat, T ; X 4. 
(From Le Maout 
and Decaisne.) 



mon nuts (Fig. 243), in which an occasional 

development of a second seed gives us the 

philopena variety. 

In many cases other parts of the flower 

persist and are incorporated with the ovary 
into the fruit, contributing to its functional effectiveness. 
Thus the style, usually deciduous with the petals and 
stamens, persists in Clematis, where it forms the very con- 
spicuous plume (Fig. 244). In the Composite family, the 



350 



A TEXTBOOK OF BOTANY 



[Ch. VII, 2 




Fig. 243. — Ripen- 
ing ovary of Buckeye, 
showing development 
of one of the six ovules. 
(After Gray.) 



so-called pappus, a structure on the ovary usually interpreted 
as morphologically calyx, persists as hooks, plumes, and 
other analogous structures (Fig. 256). Furthermore, wholly 
new structures also develop from the ovary 
wall, usually in obvious adaptation to 
dissemination. Thus many small weeds 
develop hooks or adhesive glands, making 
their ''seeds" cling tight to the clothing 
of the stroller in autumn fields. Very 
prominent are the flat wings which de- 
velop on the Maple (Fig. 245), the Elm, 
and the Ash. 

Fleshy fruits also exhibit, though less 
clearly, the signs of their origin from ovaries. They possess 
two features not found in dry fruits, — viz. bright and con- 
trasting colors, and seeds which are usually protected in 
some way against injury by digestion when eaten ; for, as 
will appear in the following section, 
fleshy fruits are eaten and their seeds 
thus disseminated by animals. The. 
simplest fleshy fruit is the berry, in 
which the wall of the ovary, whether 
carpels or receptacular cup, develops 
into the pulp, while the seeds have 
stony coats, as well exempHfied in 
the Grape, and also in Cranberry and 
Blueberry. Closely related is the 
stone fruit, or drupe, wherein the 
outer layers of the ovary wall ripen 
to the soft pulp, while the inner layers 
form the hard stone, which consti- 
tutes the most effective protection to 
the seed, as so typically illustrated in 
the Cherry, the Plum, or the Peach (Fig. 246). The fruits 
just mentioned, by the way, show on one side a depressed 
line which indicates the original joining of the edges of the 




Fig. 244. — Fruit of Clema- 
tis. (From Bailey.) 



Ch. VII, 2] 



MORPHOLOGY OF FRUITS 



351 




Fig. 245. — Fruit 
of Maple. (From 
Bailey.) 



single carpel from which each fruit is developed. In the 
fleshy fruits of the Apple and Pear type, the receptacle grows 
up and incloses the carpels (the core), forming a type called 
the POME, the receptacular nature of 
which is further attested by the obvious 
remnants of persistent sepals. In some 
of the largest gourd fruits, Hke the 
Pumpkin and Squash, the outer wall is 
hard and only the inner part becomes 
edible, while in the related Watermelon 
it is chiefly the placentae which form the 
pulp, as is likewise true in Tomato and 
Cucumber. As to the method of protec- 
tion of the seeds in large fruits like the 
Apple, Watermelon, and Orange, that 
will presently be mentioned. 

In the fruits just described the pulp results from the spe- 
cialized ripening of carpel, or receptacular ovarian wall, or 
placentoB; but it may develop from other parts also. Thus 
in the Strawberry the edible part of the fruit is wholly the 
receptacle, which bears the many seed-like 
akene fruits. In the Wintergreen berry 
the pulp is largely calyx; in the Yew 
berries it is an extra seed coat (for Yews 
have no ovaries), called an aril. In the 
Orange, which is a kind of huge berry 
with a separable skin, the pulp is con- 
stituted from hair-like structures developed 
from the inner walls of the carpels. 

In considering the various morphological 

origins of the pulp one cannot but ask why 

one plant forms it in one way and another 

so differently. As to this we have little 

exact knowledge; but circumstantial evidence indicates 

that here, as elsewhere, evolution moves along lines of least 

resistance, the pulp in any given case being made from 




Fig. 246. — Drupe 
of Cherry. The stohe 
is cross-lined. (From 
Figurier.) 



352 



A TEXTBOOK OF BOTANY 



[Ch. VII, 2 




that part which was aheady most nearly pulp-Hke in its 
structure. 

The fleshy fruits thus far described are all simple, that is, 
composed of a single pistil; but aggregate and multiple 
fruits also occur. Thus, while in Strawberry 
the pulp is the receptacle on which stand 
the many dry akenes, in the nearly related 
Raspberry the receptacle forms no part of 
the fruit, which is made up of the many 
separate aggregate carpels ripened to Httle 
drupes ; while in Blackberry both drupelets 
and receptacle are included. Further, in- 
stead of a single flower a cluster may form 
a single large multiple fruit. This is the 
case in the Mulberry (Fig. 247), in which 
the pulp is chiefly calyx, and also in the 
Pineapple, where not only the ovaries, but 
also the bracts ana main stem of a large 
cluster of flowers ripen to the single coales- 
cent fruit mass. A different form of 
multiple fruit is that of the Fig, where the flowers are 
arranged inside a hollowed receptacle (Fig. 248). Somewhat 
in the nature of a multiple fruit also is 
the cone (Fig. 249) of Pines, Spruces, 
and that family. This form of fruit 
belongs to the Gymnosperms, or naked- 
seeded plants, which have no ovaries 
but usually bear their seeds on the 
bases of overlapping scales which col- 
lectively make up the cones. 

The particular feature of pollination 
which acts as the stimulus to fruit 
formation is known. The pollen tube, 
as it reaches the embryo sac (page 278), 
contains normally two sperm nuclei, of -p^^ 243 —A Fig fruit 
which one always fertilizes the egg cell. (From Bailey.) 



Fig. 247. — The 
Mulberry, made 
up chiefly of the 
ripened calyxes of 
a cluster of flowers ; 
X I- (From Figu- 
rier.) 




Ch. VII, 2] 



MORPHOLOGY OF FRUITS 



353 



The other also enters the embryo sac, and moves towards its 
center, where it fuses with the principal nucleus of the embryo 
sac itself (Fig. 250). This fusion nucleus, with surrounding 
protoplasm, soon divides and forms the beginning of the 
endosperm or food substance later 
used by the developing embryo. 
Thus this so called " double fertiliza- 
tion" acts as the stimulus to endo- 
sperm formation, and endosperm 
formation seems clearly to act as the 
stimulus to seed and fruit formation. 
Incidentally this double fertilization 
involves another important conse- 
quence, in that the endosperm, like 
the embryo plant, contains chromo- 
somes from the pollen parent, of 
which it can thus show some char- 
acteristics. A conspicuous case 
thereof is found in Corn, where red 
grains, the result of a red endosperm 
showing through the grain coats, 
appear on white ears after pollination 
by a red variety. This phenomenon, 
called XENIA, was very puzzling until 
its real nature was discovered. 

While ordinarily a fruit does not 
form, or ''set"' in the gardener's 
phrase, unless fertilization has oc- 
curred and an embryo is formed, cases 
are known in which the fruit develops without the presence 
of embryos in the seeds, a condition called parthenocarpy. 
In most such instances pollination is essential, as indeed 
would be expected from the role of double fertilization ; but 
in a few cases, notably some Figs, even polhnation is not 
essential, and fruit formation follows on flower formation 
without any known special stimulus. 
2a 




Fig. 249. — A typical 
cone, of Fir (Abies pecti- 
nata) ; reduced. 

Above, on right, a scale 
with ovules ; on left the 
seeds, with part of the scale 
separating with them to 
form "wings." (From 
Sachs.) 



354 



A TEXTBOOK OF BOTANY 



[Ch. VII, 2 



The development of ovaries into fruits involves often a 
great increase in size, as notable in the gourds. Herein is 
involved not simply an enlargement of cells already present 
in the ovary, but abundant new cell formation from the 

parenchymatous, but not spe- 
cially meristematic, tissue, — a 
method identical with that by 
which the bark of trees and 
the chlorenchyma of leaves are 
enlarged. 

While the ovary is develop- 
ing to the fruit, the ovule is 
developing to the seed. The 
coats of the ovule harden, 
with some changes, to the seed 
coats; the micropyle becomes 
sealed with corky tissue; the 
formation of the endosperm 
tissue is completed ; and (most 
important of all) the fertilized 
egg cell develops into the 
embryo plant. 

The development of the 
embryo from the fertilized egg 
cell may best be traced in a 
typical case, illustrated in Fig- 
ure 251. First the egg cell 
divides, and then the resultant 
cells divide, for a time in a 
line, forming a suspensor, the 
end cell of which, thus brought 
well out into the embryo 
sac, forms the new embryo. This so-called initial cell 
divides, as shown in our figure, and divides again until 
there is formed a multicellular globular structure. Then 
growth becomes more active at special points, there forming 




Fig. 250. — Double fertilization 
in the embryo sac of Lilium Mar- 
tagon, generalized. 

One of the two sperm nuclei, spi, 
is shown in contact with the nucleus 
of the egg cell, ov ; the other sperm 
nucleus, sp2, is in contact with the 
embryo sac nucleus, ek. The cells 
si and S2, called synergidse, and the 
three marked a, called antipodal 
cells, represent an inheritance of 
the thallus of lower plants. (After 
Strasburger.) 



Ch. VII, 2] 



MORPHOLOGY OF FRUITS 



355 



the ''seed-leaves" or cotyledons and the hypocotyl, as 
shown in our figure. At this stage the embryo consists 
mostly of meristematic tissue, though an epidermis is well 
formed, and the fibro-vascular system begins to appear, while 




( 



Fig. 251, — Stages in the development of a typical embryo, of Rape 
(Brassica Napus) ; X 250, 

The embryo sac, in part, is shown on the left ; the egg cell was at its lower 
end, and has grown into the suspensor, with the initial cell at the top. Above, 
left, the 8-celled stage of the embryo, with suspensor, shown with protoplas- 
mic contents. Below, middle, are cross sections of the globular stages of the 
embryo. Right, a nearly formed embryo, with two cotyledons, the sus- 
pensor not yet absorbed. (After L, Kny.), 

the foundation for a root develops next the suspensor. 
Meantime the endosperm is developing until it surrounds 
the growing embryo, the suspensor becoming absorbed; 
and the two together finally fill the embryo sac. This 
is the state of the embryo in some seeds (page 374) when 
ready for germination, but in other kinds the embryo con- 



356 A TEXTBOOK OF BOTANY [Ch. VII, 3 

tinues to grow and (by action of digestive enzymes) absorbs 
the endosperm, and even the nucellus, so that finally it comes 
to fill completely the seed coats and contains all of the seed 
food within itself. The shape of the embryo is correlated 
with that of the embryo sac, being short and straight in 
some, variously elongated and curved or bent in others, often 
to extreme degree. Whatever the shape, however, it fol- 
lows from the mode of development of the embryo that 
the root end of the h;^pocotyl lies next to the micropyle. 

3. The Dissemination and Dispersal of Plants 

Many features in fruits show obvious connection with the 
dissemination of seeds, though fruits are by no means thus 
completely explained. Properly, dissemination means sim- 
ply the scattering of seeds from the parent plant, but the 
term is often employed more broadly, even to an equivalence 
with dispersal, which means the spread of plants over 
the earth's surface. Many parts besides fruits are here 
concerned, so that we may best consider the subject as 
a unit. 

That plants which have in themselves no power of free 
locomotion such as animals possess can yet spread very 
widely and quickly is shown by the familiar case of weeds, 
which, introduced into a new territory, often overrun it 
before man becomes aware of the dai^^er. A striking pres- 
ent-day instance is familiar in the deadly Chestnut disease, 
a Fungus with wind-carried spores, which, introduced from 
Asia into eastern America about ten years since, has already 
srpread through several states. Animals could hardly 
spread faster. 

The physiological necessity for some method of dissem- 
ination is amply obvious, for if all spores or seeds produced 
by a plant were to germinate where formed, or on the ground 
directly beneath, the resultant congestion would prevent 
normal development of any of the plants. Green plants 
need room to spread 'their foliage to the sun, and parasites 



Ch. VII, 3] DISSEMINATION OF PLANTS 357 

need new host plants ; so that plants, in a manner like 
animals, must spread in search of food. Accordingly, dis- 
semination is not a mere incident of plant life, but a neces- 
sary function. 

The general methods of plant dissemination and dispersal 
fall under some six heads as follows : 

1. Independent locomotion. While none of the familiar 
land plants have any such power ^ it is frequent in the lower 
Algae. Thus, as will appear more fully in Part II of this 
book, some of the simplest kinds work their way over the 
bottom of ponds by aid of protoplasmic threads, while others 
jerk their bodies along by sudden vibrations. More famil- 
iar and typical, however, are the zoospores possessed by 
many Algae (page 301, Fig. 213), which can swim by action 
of vibratory cilia in a manner so anirhal-like as to have 
originated their name. The Slime Molds, or Myxomy- 
cetes (page 38, Fig. 14), living out of the water but in damp 
places, can creep over wet surfaces in a manner identical in 
aspect and method with that of the animal Amoeba. 

2. Extension through growth. As described in earlier sec- 
tions (pages 187-9), many plants send off stems along the sur- 
face of the ground or beneath it, in the forms called stolons, 
offsets, runners, and rootstocks, which take root at their 
tips and there form new plants, after which the old con- 
nection with the parent often withers away. The Straw- 
berry offers a familiar and typical example of this mode 
of spread ; the suckers which spring from the ground in the 
vicinity of fruit trees, often from old roots, are other ex- 
amples; but it reaches perfection in the Grasses, especially 
the familiar Couch Grass of the gardens. Of course all 
such plants have likewise a dissemination by their seeds, 
their spread by growth-extension being additional and often 
incidental. In fact all gradations in this method are 
found from cases clearly incidental or accidental up to 
those which seem clearly adaptational, in which fact is prob- 
ably embodied a leading principle of evolution, viz. that 



358 



A TEXTBOOK OF BOTANY 



[Ch. VII, 3 




Fig. 252. — Fruit 
of American Elm, 
containing one seed ; 
enlarged. (From 

Bailey.) 



adaptational features frequently, perhaps mostly, arise by 
the development of features originally incidental. . 

3. Waftage by winds. This is the commonest of the 
methods of dissemination, and the most efficient for wide 
dispersal. Fruits and seeds often develop 
a large spread of surface in proportion to 
bulk, thus giving a hold to the wind which 
wafts the seeds to a distance. Often this 
feature is a ''wing," a thin flat plate de- 
veloped from the ovary, as in Maple or 
Elm (Fig. 252), or from the seed-coat, as 
in Bignonia (Fig. 253), or from part of a 
cone scale, as in Pine (Fig. 249), or from a 
bract, as in Linden (Fig. 47). That the 
plants here mentioned are all trees is not 
mere coincidence but typical of the fact 
that winged fruits or seeds are almost 
confined to trees or high-climbing shrubs, while the same 
is true of kinds having very flat pods, as in Locusts. 
On the other hand, the fruits and seeds of herbs and low- 
growing shrubs more often have hairs or plumes, either in 
great profusion, as in Cotton (Fig. 254), or in terminal tufts, 
as in Milkweed (Fig. 255), or in parachute-like arrange- 
ments, as in Dandelion (Fig. 256) ; 
or in yet other ways. Such structures 
give a hold to the wind, which per- 
mits not simply a lateral transport 
of the seeds before they reach the 
ground, as occurs and suffices in the 
case of trees, but a lifting action 
whereby even light breezes raise the 
seeds to a height whence winds may 
carry them far. It is notable that plants of this kind, as 
witness the Dandelion, are among the most widespread and 
abundant of plants. The plumes are of diverse morpho- 
logical origins, being special outgrowths from seed coat 




;K:!3>^ 



Fig. 253. — Seed of Big- 
nonia alba-lutea. Reduced. 
(From F. Darwin.) 



Ch. VII, 3] DISSEMINATION OF PLANTS 



359 







Fig. 254. — Seed of Cot- 
ton, with the important 
. long hairs; X {. (From 

Ine Kose oi Jericho, or Figurier.) 



(Cotton), ovary (Willow), calyx (Compositse), persistent style 
(Clematis), and aborted flower stalks (Smoke Bush). Here 
also, it is likely, the general principle of modification along 
lines of least resistance explains the 
particular cases. 

Another mode of wind dissemina- 
tion is found in the "tumble weeds." 
In these the flower cluster, as in some 
members of the Parsley Family, or 
else the entire plant, as in Russian 
Thistle, becomes detached, incurls 
the branches, and thus is rolled over 
open ground, scattering the seeds as 
it travels. Such plants are espe- 
cially characteristic of open plains 
country. 

the Scriptures (Fig. 257), is said to 

have this habit, thus giving a concrete meaning to the scrip- 
tural phrase ''blown hke stubble before the wind." 

It is also the wind which scatters, though indirectly, the 
small, rounded, smooth seeds formed in so many kinds of pods 
upon long stalks, such as the Poppy (Fig. 239) ; for such seeds 

seem to be shaken forcibly 
from the pods when struck by 
strong gusts in autumn and 
winter. It has been claimed 
that the pods have such form 
and angles of exit as to guide 
the seeds well away from the 
plant. 

Finally (in so far as we can 

take space to discuss this phase 

of our subject), the wind effects 

dissemination of very minute spores or seeds without any 

special arrangements. The method rests on the fact that 

as a body decreases in size, its bulk diminishes far faster 




Fig. 255. — Seed of Milkweed 



360 



A TEXTBOOK OF BOTANY 



[Ch. VII, 3 




Fig. 256. — Fruit of 
Dandelion, containing 
one seed ; X 2. (Frona 
Le Maout and De- 
caisne.) 



than its surface, on which account very minute bodies present 
so much surface in proportion to their weight that the hghtest 
breezes can waft them into the air and 
carry them indefinitely. This is why 
dust floats in the air, and with the dust 
float the very minute spores of innumer- 
able plants, of Ferns, Mosses, and many 
kinds of Fungi, — Mushrooms, Molds, 
Mildews, Blights, and the resting spores 
of Yeasts and Bacteria ; and thus is ex- 
plained the remarkable spread of those 
ubiquitous organisms. The seeds of 
some tropical Orchids are small enough 
to be spread by this method, especially 
as their surface is enlarged by presence 
of small bladders. 

Dissemination is the basis of dispersal, 
in which the wind is the most effective 
agent. Thus small plumed seeds as well as spores, which 
can be lifted by light breezes, are carried over vast terri- 
tories by great gales, even from island to island, and conti- 
nent to continent. 

4. Flotage by water. The currents of rivers and oceans are 
of course important agencies of dissemination and dispersal. 
This result is often incidental, as when 
twigs of Willows, or even small plants 
washed out in time of flood, are carried 
down stream and left to take root on some 
emerging bank. Many fruits or seeds 
ordinarily scattered by wind float also on 
water, and thus are carried by rivers, 
which transport likewise the separable 
winter buds of many Water weeds. There ^^^ 257 — The 
are striking cases in which seeds, them- Rose of Jericho 
selves heavier than water, possess arrange- ^J!tt7ca)T'''y.^''''^ 
ments whereby they are kept buoyed up (From Bailey.) 




Ch. VII, 3] DISSEMINATION OF PLANTS 



361 




Fig. 258. — Seed of 
Water Lily, with air- 
holding coat (aril) . 
(After Gray.) 



on its surface. Thus the White Water Lily seed has a 
loose, air-holding extra coat (Fig. 258) and some Sedges 
have loose-textured carpels, which form efficient floats. 
Some seeds have flat, corky expansions, as in Iris, while 
others float by virtue of the unwettable 
surface they possess. The Nelumbium 
(the Lotus of ancient literature and art) 
has a broad, buoyant receptacle (Fig. 
259), out of which the seeds are said to 
be dropped from time to time in its 
decay. Very striking is the case of the 
Coconut (Fig. 260), in which the ovary 
develops to a great air-holding husk com- 
posed of material specially resistant to salt 
water, with which arrangement is correlated the fact that this 
plant has been carried by ocean currents and the winds all 
around the tropical seas. Ocean currents, indeed, contest with 
winds for the first place among agencies of plant dispersal. 

5. Carriage by animals. Very important as agents of 
dissemination, and even of dispersal, are animals, which are 

effective in two different 
ways. 

First, many fruits or seeds 
possess structural arrange- 
ments by virtue whereof they 
are held attached to the bodies 
of animals, and are finally 
dropped far from their places 
of formation. Especially com- 
mon are hooks, which become 
entangled in wool or fur, 
where they often remain until 
the hair is shed. Such hooks 
origins ; they are developed 
bracts of the flower head in Burdock (Fig. 261), and out- 
growths from the ovary wall in Cocklebur (Fig. 262). 




Fig. 259. — Floating receptacle of 
Lotus (NelumbiuTn) ; x h 



have diverse morphological 



362 



A TEXTBOOK OF BOTANY [Ch. VII, 3 




Sometimes they are gigantic in size, as in the Martynia of 

the western plains (Fig. 263). Everybody knows how abun- 
dantly the weed seeds cling to our 
clothes after walks in the fields in the 
autumn ; and they cling mostly by 
hooks. The same result follows from 
the presence of adhesive coverings to 
fruits or seeds, as especially common in 
epiphytes or parasites, for instance, 
the Mistletoe. In these cases the 
sticky seeds adhere firmly to the feet 
of perching birds until brushed off by 
contact with rough parts of some 
tree, the adhesiveness then serving to 
attach the seeds to the tree upon which 
they must grow. Adhesive seeds occur 
also in some water plants, which thus 
become attached to the feet or feathers 
of wide-ranging water birds, many of 
which travel so widely as to render 
those plants cosmopolitan. Thus birds 
come next after winds and ocean cur- 
rents as agents of plant dispersal. 

Second, fleshy fruits, with their edible, palatable pulp and 

their bright, contrasting colors, are 

easily found and eaten by animals, 

through the bodies of which the seeds, 

variously protected against injury from 

digestive juices, pass uninjured, and 

thus are dropped far from their places 

of origin. This seems very clearly the 

functional significance of edible colored 

fruits in nature, all lines of evidence 

converging upon this explanation. In 

this way the smaller forms of fleshy fruits, especially the 

diverse forms of berries and the smaller drupes, are scattered 



Fig. 260. — 
nut, in section 

It shows the air-hold- 
ing husk, the hard shell 
(black), endosperm or 
"meat" (cross-lined), 
and central cavity con- 
taining sap, or "milk." 
Below in the endosperm 
can be seen the small 
embryo, which lies just 
under, and comes out in 
germination through one 
of the "eyes." (From Le 
Maout and Decaisne.) 




Fig. 261. — Head of 
Burdock fruits ; X y. 
(From Kerner.) 



Ch. VII, 3] DISSEMINATION OF PLANTS 



363 




Fig. 262. — Fruits of 
Cocklebur ; X h- (From 
Kerner.) 



chiefly by birds, which are the most important disseminators 
of seeds by this method. The larger fruits, Apples, Oranges, 
Gourds, and multiple fruits, are eaten in 
part by birds and in part by mammals. 

The pulp has very diverse morpho- 
logical origins, as earlier noted (page 351), 
and the protective coats to the seed are 
either the seed coat or else a pit formed 
from ovary wall (page 350). The claim 
has been made that the seeds of the 
largest fruits (those too large to be swallowed whole) mostly 
find their protection in the slipperiness of the coats, which 
prevents their being actually swallowed at all, though they 
are carried some way with the fruit. 

Some seeds have bright colors without pulp, as in Abrus 
beans, and those of the common Magnolia; and such are 
said to be swallowed by birds, which perhaps mistake them 
for something edible, or else simply desire their bright 
attractiveness, as trout take the sportsman's fly. Some 
seeds and fruits which bear striking resemblances to insects 

are supposed by some ob- 
servers to secure dissemi- 
nation on that account, 
though this is not certain. 
Recently it has been claimed 
that the many small seeds 
provided with caruncles, 
i.e. little rounded projec- 
tions filled with nutritive 
food substances (Fig. 270), 
are very effectively dis- 
seminated by ants, which 
carry off the nutritive bodies, and incidentally the seed 
therewith. Also snails and even fishes have been claimed 
to carry seeds. 

The greatest results in the dissemination and dispersal of 




Fig. 263. — Seed pod of Martynia lutea; 
X h. (From Le Maout and Decaisne.) 



364 



A TEXTBOOK OF BOTANY [Ch. VII, 3 



plants are accomplished by man himself, who carries plants 
deUberately all around the world for his own purposes, 
and also spreads them accidentally through his commerce 
and travel. Much of his deliberate dispersal has effect 
only so long as his watchful care persists, for without it the 
most of his carefully grown crops would soon be exter- 
minated by the return of the native vegetation. 

6. Projection by spring -release. In the dissemination of 
plants by aid of winds, water currents, and animals, the 

fruits and seeds are 
wholly passive, and 
the results are secured 
by the presence of 
structures which make 
the dissemination an 
inevitable result of the 
natural and ordinary 
operations of those 
agencies. There is, 
however, one way in 
which plants effect an 
active dissemination 
by forces developed 
within themselves, — 
namely, in the hurling 
of seeds through the 
air by the action of suddenly released spring mechanisms. 
Thus in Violets (Fig. 264), the pod so ripens that the carpels 
press harder and harder upon the smooth seeds held in 
an angle between them, until suddenly the pressure over- 
comes the friction and the seeds are shot to a distance, 
much as one may shoot a smooth bean from between the 
pressed fingers. Again, in the Vetches, bands of tissue in 
the pod so ripen under tension as to bring such a strain 
on the sutures of dehiscence that they suddenly rupture 
and shoot the seeds forth in every direction (Fig. 265) » 




Fig. 264. — Seed pods of a Violet, forcibly 
projecting the seeds. (From Kerner.) 



Ch. VII, 3] DISSEMINATION OF PLANTS 



365 




The Witch Hazel hurls forth its seeds in similar manner, as 
does the Acanthus, the Castor Bean, and the West Indian 
"Sand box," which bursts with a 
noise described as like a pistol shot. 
In some fruits it is the ripening not 
of dry, but of osmotically turgescent 
tissues, which produces the explosive 
result, as in case of the Impatiens, 
called also descriptively " Touch- 
me-not," and the so-called '^Squirting 
Cucumber" of the Levant (Fig. 266). 
Many accessory adaptations in 
fruits, connected more or less directly 
with dissemination, have been de- 
scribed by various observers, although 
not always accepted as genuine by 
others. The Kenil worth Ivy and the 
Chinese Primrose both grow in their 
native homes upon steep rocky hill- 
sides or chff s ; both have phototropic ^f trowing its seeds. 

' *■ '- (iTom Kerner.) 

flowers, but their seed pods turn 

away from light, thus bringing the seeds into clefts of the 
rocks, whereby the plants are kept at the same elevations. 

Dandelions and 
c^-^^-Si'- /^ other plants hold 

their flower buds 
close to the 
ground until 
their flowers are 
ripe, then raise 
them until after 
pollination, then 
draw them down 
once more until 

Fig. 266. — The Squirting Cucumber {Ecbalium *"® iruits are 

Eiaterium). ripe, and then 



Fig. 265. — A pod of 
Lathyrus vernus, bursting 




366 A TEXTBOOK OF BOTANY [Ch. VII, 4 

raise the plumose fruits well up into the winds ; and water 
plants have analogous ways of drawing their pollinated 
flowers under water, and later releasing the ripened fruit or 
seeds (page 288) . Several minor parts in connection with 
dissemination have been ascribed to hygroscopic tissues, 
with their power of strong movement (page 237), including 
the sudden release of elastically set tissues, certain creeping 
movements of fruits along the ground, and others, most of 
them probably only incidental. 

Nowhere among plants, excepting in connection with cross- 
pollination, appear so many features of structure which 
have been interpreted as adaptations. Our sketch of the 
subject does it little justice, but is all that our space per- 
mits. The student can find ample additional detail in the 
books especially devoted to this subject. 

4. Special Forms and Monstrosities of Fruits 

Fruits, like flowers, but unlike leaves, stems, and roots, 
seem not to perform functions other than the one which 
is primary and typical to them ; and conversely there seem 
to be no special organs of plants which can be traced to a 
morphological origin in a transformed fruit. But of ab- 
normalities and monstrosities, fruits show a good many, 
mostly having connection with similar features in flowers. 
They have largely been treated, however, in earlier sections, 
and need only be reviewed at this place. 

Mechanically caused effects, simulating monstrosities, are 
found in Strawberries or Raspberries, where one side of the 
fruit remains hard and green as result of a failure of com- 
plete pollination (page 197) ; and an Apple or an Orange dis- 
playing a clean-cut segment of different skin is a chimcera, 
resulting from grafting (page 211). Some twin fruits are 
also a product of natural grafting when very young, soft- 
tissued, and tightly pressed together (page 196), though some 
twin fruits, as in Partridge Berry, are perfectly normal and 
usual. 



Ch. VII, 5] NATURE OF PLANT DISEASES 367 

Among genuine monstrosities, twin fruits, which are rather 
common, result from a partial fission of one (Fig. 146 and 
page 198). A genuine fasciation, producing several distinct 
lobes, occurs, in highly cultivated Strawberries, and also 
in Pineapples, as earlier pictured (page 198). Two-storied 
fruits also occur as result of proliferation (page 202) , though 
sometimes a leafy branch occurs in place of the second fruit ; 
and the Navel Orange is an imperfect case. Apples and 
Cucumbers occasionally produce bracts on their sides (page 
202). Sometimes the carpels do not all unite in the pistil, 
but some remain separate, as is said to occur rather often in 
Oranges. Ears of Corn sometimes have tassels of male 
blossoms on their upper ends, and sometimes branch at the 
base. And other monstrosities less common are met with 
of which some can be propagated. 

5. The Nature and Cure of Plant Diseases 

Plants are subject to many diseases, which appear perhaps 
in fruits more prominently than elsewhere. 

Plant diseases, which are studied very thoroughly in agri- 
cultural institutions under the name of Phytopathology, 
or simply Pathology (page 4), fall into three general cate- 
gories, — those caused by parasitic Fungi, those of purely 
physiological origin, and those of which the nature is still 
unknown. 

Parasitic Fungi, including Bacteria, cause the great ma- 
jority of the worst plant diseases, notably Rusts, Blights, 
Smuts, Mildews, Wilts, Rots, Cankers, Crown Galls, Black 
Knots, Scabs, Spot and Blotch diseases, and other special 
diseased growths, the description of which will be found in 
Part II of this book. The very small spores by which these 
Fungi reproduce are carried everywhere by winds, and when 
thus brought to a suitable host plant they germinate, and 
the haustorial threads enter the plant body either through 
the stomata, the water pores, or some injury of the surface, 
or even at times, though rarely, directly through the epi- 



368 A TEXTBOOK OF BOTANY [Ch. VII, 5 

dermis. Then the feeding threads, or myceha, branch every- 
where throughout the tissues of the host, which they pene- 
trate by aid of enzymes secreted by their tips; while the 
walls and protoplasm and foods thus digested are absorbed 
and used in the further growth of the mycelium. Sometimes 
the Fungus takes no great toll and the host continues to live ; 
but often the damage proceeds even to the death of the host. 
It is not solely, often indeed not principally, by the robbery 
of food that the parasite works the injury, but by poisonous 
substances, by-products of its metabolism, released in the 
tissues of the host. Thus parasitic Bacteria release into 
the bodies of animals, including man, the poisonous toxins 
which are the real cause of death from bacterial diseases 
(page 173) ; and in analogous manner parasitic Fungi cause 
damage and death to large areas of tissues, with resultant 
formation of brown or black spots, which often spread until 
the host's nutrition falls below the necessary minimum, and 
it dies. In addition, Fungi also cause a kind of paralysis 
of the growth-control mechanism of their hosts, resulting 
in the formation of many kinds of monstrosities (page 199), 
especially diseased woody growths. In some cases the Fun- 
gus attacks parts which are not vital, such as the heart wood 
of trees ; but others attack the most essential parts, as in 
case of the Chestnut disease, where the Fungus destroys the 
living cambium. In any case the Fungus sooner or later 
sends out to the surface of the host its spore-producing struc- 
ture, from which the ripe spores are wafted by the wind, this 
*' fructification," as it is sometimes called, being often the only 
part of the parasite visible on the outside of the host. 

Physiological disturbances include sun scalds, where the 
sensitive tissues are injured by too much sunlight ; oedema, 
an osmotic disease (page 234) ; chlorosis, or failure of 
chlorophyll to develop, producing unhealthy mottled leaves ; 
and a number of others. One might almost include death 
from old age in this category, although plants differ much 
from animals in this relation (page 114). 



Ch. VII, 5] NATURE OF PLANT DISEASES 369 

Unknown causes, apparently not parasites or any known 
kind of physiological disturbance, produce the important 
diseases known as Peach Yellows, the Mosaic Disease of 
Tobacco, the Curly Top of Sugar Beets, and others, these 
names sufficiently describing the characteristic symptoms. 
Perhaps a wholly new type of disease-cause awaits dis- 
covery. 

It is not usual to include insect ravages among plant 
diseases, but rather to reckon them in a category of pests, 
although their effects come often very close to those of the 
true diseases, and they are combated by much the same 
methods. Insects do damage in three principal ways : 
first, they eat the leaves of plants of which the food-making 
power is thus damaged. Second, they bore into and feed 
upon fruits or other parts, which thereby are rendered re- 
pulsive even if not extensively injured. Third, they lay their 
eggs in plant tissues under conditions which promote the 
formation of galls, already described (page 203) . Insects are 
chiefly combated by poisons, either in sprays, as against the 
Elm Beetle, or else in gas, as in the fumigation of green- 
houses, or of fruit trees under specially made tents. 

The great number, variety, and ubiquity of plant diseases, 
together with the excessive loss they entail every year upon 
agricultural and horticultural interests, have long stimulated 
their practical study, with a resultant development of elab- 
orate methods for combating them. In the past these 
methods have mostly centered in the effort to find a sub- 
stance fatal to the parasite and not to the host, and such is 
the principle of spraying plants with solutions like the well- 
known Bordeaux mixture. Another method, of which the 
importance has more recently become manifest, consists in 
determining and applying to plants the conditions requisite 
for keeping them in fullest vigor, for the condition of bal- 
ance between parasite and host is often such that a healthy 
individual can resist while a weakened one cannot. This 
principle becomes clear in connection with the ventilation 
2b 



370 A TEXTBOOK OF BOTANY [Ch. VII, 6 

of greenhouses or storehouses for fruits or root crops. Fungi 
as a rule, in correspondence with their small size and para- 
sitic mode of life within tissues, can stand bad air much bet- 
ter than the higher plants, which are far larger, and physi- 
ologically adjusted to more space and air. Accordingly, 
bad ventilation always gives to Fungi an advantage over 
their hosts, which explains why plants rot more freely in 
un ventilated than ventilated places. 

In our own times an attack has been made on the disease 
problem along a very different line, viz. the effort to breed 
immune varieties. The variability of plants is manifest in 
susceptibility to a given disease as in other features, some 
individuals of a kind being very susceptible and others much 
less so. By a systematic selection of the latter individuals 
it is found possible to breed immune races. Though the 
problem is by no means so simple in practice as in prin- 
ciple, marked success has already attended the method, of 
which we are sure to hear more in the future. 

6. Economics, and Cultivation, of Fruits 

The economic uses of fruits, apart from their seeds, which 
will be considered later by themselves, center almost wholly 
in the food value of the edible kinds, especially berries, 
pomes, and gourds. It is notable, however, that fruits as 
food are rather luxuries than necessities, having a relatively 
slight food value though great palatability. This com- 
bination, of course, comports perfectly with their function 
of seed dissemination by agency of animals, where the 
function does not require that animals shall be nourished, 
but only attracted. The dry kinds of fruits have hardly 
any uses to man, the opium derived from young Poppy pods 
being perhaps the most prominent economic product. 

As to the cultivation of fruits, the physiological and struc- 
tural methods concerned have mostly been covered inci- 
dentally in the earlier sections. Such are the pollination of 
flowers by insects to insure fruit setting ; pruning to divert 



Ch. VII, 6] ECONOMICS OF FRUITS 371 

plant energy from superfluous vegetation to optimum fruit 
formation ; grafting to preserve and multiply a desirable new- 
variety or sport ; spraying and other like methods, including 
the use of poisonous gases, to combat parasitic fungi and 
insects ; plant breeding for the development of better or im- 
mune new kinds; and ventilation, of greenhouses or store- 
houses, to aid the larger plants and fruits in resisting dis- 
ease Fungi. Fine fruits are commonly wrapped individually 
for transport, chiefly to prevent the breaking of the skin, 
which not only injures their appearance, but breaks their 
first line of defense against Fungi. 



CHAPTER VIII 
THE MORPHOLOGY AND PHYSIOLOGY OF SEEDS 

1. The Distinctive Characteristics of Seeds 

Seeds form the transportation stage by which plants, 
otherwise sedentary, are spread outward from their places 
of origin. They are relatively small parts, which separate 
of themselves from the parent plants, are so constructed that 
they may be kept long alive and carried afar, and then 
when planted, produce individuals like those which produced 
them. Living under diverse conditions, and transported 
by various agencies, they differ much in their various ex- 
ternal features. 

In size, seeds vary from almost dust fine in some tropical 
Orchids (much smaller than the scriptural mustard seed), 
all the way up to the Coconuts, the mean or average size 
lying, however, much nearer the former than the latter 
dimensions. 

In shape, seeds are most diverse, from approximately 
globular to elongated, variously angled, and extremely flat, 
the particular shape being apparently connected in some 
cases with the method of dissemination, in others with the 
shape of the embryo, and in others with less evident factors. 
Often the shape is complicated by the development of 
special outgrowths, — plumes, hooks, etc., — having con- 
nection, as in case of fruits, with dissemination. 

In color, seeds are sometimes bright, chiefly red or yellow, 
as are some fruits, and in other cases are mottled, sometimes 
in ways which simulate the markings of insects ; and all of 
these features may have connection with dissemination 

372 



Ch. VIII, 2] MORPHOLOGY OF SEEDS 373 

(page 356). Far more common, however, are the simple 
brown and gray colors such as all ripening tissues assume, 
as a purely chemical incident, where no functional reason 
for special color exists. This is the case with all wind- and 
water-disseminated seeds, and those thrown by spring 
mechanisms. 

In texture, seeds are almost invariably hard, sometimes 
extremely so, as in the Ivory Palm. The hardness results 
from three causes : the almost complete absence of water 
from the tissues, the very thick coats, and the frequent 
large proportion of cellulose food. They soften very greatly, 
however, in germination. 

Many seeds are difficult to distinguish externally from 
small fruits, especially when the latter are one-seeded. 
The difference is purely morphological, consisting in the 
presence of the ripened ovary as a kind of additional coat; 
but functionally such one-seeded fruits are identical with 
true seeds. 

As with other plant organs, there are various structures, 
popularly thought to be seeds which are not, such as the 
little black, hard-walled bulblets in the axils of Lily leaves 
(Fig. 212). There is no such thing as ''Fern seed," the mis- 
named ''fruit dots" on the under sides of fern fronds con- 
taining only spores, a very different kind of structure. Spores 
have no embryo, that is, no ready-formed young plant, which 
all true seeds contain. Indeed the possession of an em- 
bryo is by far the most distinctive mark of a seed. 

2. The Structure, Morphology, and Functions of 

Seeds 

Despite a wide variety in their external features, seeds 
possess in common certain primary parts, three and some- 
times four in number, which of course are those essential 
in their function. 

First in importance is the embryo, which is simply a 
formed but unborn plant, with its development held for a 



374 



A TEXTBOOK OF BOTANY [Ch. VIII, 2 




Fig. 267. — Embryo 
of Lima Bean, as it 
appears without the 
seed coat, and on re- 
moval of one cotyle- 
don ; X J. The largest 
part is cotyledon. 



time in suspension. It has always a small unjointed stem, 
the HYPOCOTYL, which bears at one end the foundation for 
a root, at the other the foundation for a bud, and close 
to the latter one or more ''seed leaves," 
or COTYLEDONS (Figs. 267-9). These 
cotyledons, which vary from leaf-thin to 
hemispherically thick, are oftenest two, 
less frequently one, and sometimes 
several, as prevails in the Pine family. 
While the cotyledons are commonly 
viewed as morphologically leaves, modi- 
fied by their peculiar position and func- 
tions, there is doubt as to whether they 
really originated as leaves, or are the 
descendants of special organs by which 
embryos originally absorbed food direct 
from the parent plants. The bud be- 
tween the cotyledons is mostly undeveloped in the seed, 
but in some large, well-developed embryos, it produces visible 
leaves, in which case it is called the plumule (Fig. 267). 

Second in importance is the nutritive food, which is sup- 
pUed by the parent plant, and consists chiefly of starch, oils, 
and proteins, in a dry and concentrated condition. In some 
kinds this food occurs in a special tissue, 
called ENDOSPERM, surrounding the em- 
bryo, which therefore lies embedded 
within it (the so-called albuminous 
seeds. Fig. 270) ; but in others the food 
is contained inside the embryo itself, 
mostly in the cotyledons which are then 
conspicuously enlarged ; and the embryo 
now completely fills the space within the 
seed coats (ex- albuminous seeds. Fig. 271). The endosperm 
originates in the embryo sac simultaneously with the embryo 
(page 354), and the two develop step by step together until 
they fill the embryo sac, and even (through the absorption 




Fig. 268. — Embryo 
of Morning Glory, with 
one cotyledon removed; 
X 2. There is no 
plumule. 



Ch. VIII, 2] 



MORPHOLOGY OP SEEDS 



375 



of the nucellus by the endosperm) all of the space within the 
seed coats. Such are the albuminous seeds, in the germi- 
nation of which the embryo absorbs the 
endosperm through its cotyledons. In 
the ex-albuminous seeds, however, this 
absorption of the endosperm occurs 
before germination, and this is the mean- 
ing of the difference between the two 
kinds. It is in correlation with this 
further stage of development that ex- 
albuminous seeds have so often a 
plumule, while albuminous kinds have ^o^?- ^^ longitudinal 

1 1 1 1 in 1 • ^ section ; X 3. 

only the undeveloped foundation of a 
bud. 




Fig. 269. — Grain of 



At the right is the 
embryo, showing plu- 
mule, primary root, 

Third of the parts are the seed coats, and hypocotyi. in the 
Oftenest there is but one, which is thick, if^**^^ «^^ ,^^ seen the 

' ' nbro-vascular system 

hard, and woody, and has the obvious extending into the large 
function of protecting the embryo against scutellum which 

/ ^ . forms a haustorial or- 

injury during the period of dissemina- gan for absorbing the 
tion. Sometimes there is also an inner endosperm, -et^ (looser 

texture) and eg (more 
coat, then usually compact texture). It 

membranaceous, and ^^ doubtful whether the 

' cotyledonis represented 

less often an addi- by the scutellum, by 

tional outer coat, *^^ sheath leaf of the 

' plumule, or by both to- 

called an aril, which gether. (From Goebel.) 

is generally loose 
from the others and has obvious con- 
nection with dissemination, as in cases 
earlier mentioned, i.e. the Yew berries 
(page 351) and the Water-hly seeds 

nous seed, of Castor (p^^g^ 3^^)^ rpj^^j.^ jg ^^^^ structural 

iSQSiHj ID. SGClIOH , /\ ^. 

The embryo lies em- Connection, uot yet fully understood, 
bedded in endosperm; between these arils and the httle insig- 

below is a caruncle. ^ _ *^ 

nificant and seemingly functionless swell- 
ing called the strophiole, occurring near the hilum in 
some seeds, and the much larger caruncle (Fig. 270), an 




Fig. 270. — Albumi- 



376 



A TEXTBOOK OF BOTANY [Ch. VIII, 2 




Fig. 271. — Ex-albu- 
minous seed, of Apple ; 
X4. 

The embryo, show- 
ing clearly the coty- 
ledons and hypocotyl, 
fills completely the 
space inside the seed 
coats. 



appendage which contains nutritive substances apparently 
having a functional meaning in connection with dissemi- 
nation (page 356). 

Seeds show many structural relations 
with the ovules from which they develop, 
precisely as do fruits with their ovaries, 
though it must not be inferred that all 
such features in seeds and fruits are 
simple persistences of ovule or ovary 
characters. It is equally possible that 
some have originated in seeds or fruits 
and worked back in evolution into ovules 
and ovaries. 

Every seed shows on its coat a tiny 
pit, sometimes differently colored, which 
is the persistent though now sealed 
MiCROPYLE, or opening through which 
the pollen tube entered the ovule (page 278). This of 
course has no connection with the much larger scar, called 
the HiLUM, left where the seed breaks away from its stalk 
(Fig. 272). Where ovules are turned over on their elongated 
stalks, which are grown to the coats (page 
272), the arrangement persists, in the seeds, 
which show a marked ridge, or raphe. The 
position of the chalaza of the ovule often 
is manifest in a marked chalazal angle in 
the seed. 

Appendages, when present, whether hairs, 
plumes, hooks, or others, are direct out- 
growths from the seed coat, and have 
obvious function in connection with dis- 
semination, as already discussed (page 356). 
Outgrowths of the same kind occur often 
on ovaries which contain only a single seed 
in which case one can tell only by dissection whether an 
ovary wall is present or not. 




Fig. 272. — Seed 
of a pansy ; X 5. 

Below and facing 
to the left is the 
hilum ; at the point 
(invisible) is the 
micropyle ; along 
the side on the left 
is the raphe ; and 
at the top is the 
chalazal angle. 



Ch. VIII, 3] VITALITY OF SEEDS 377 

Seeds apparently present no transformations into struc- 
tures of other function, and few abnormalities or monstros- 
ities. The principal peculiarity of this kind consists in 
POLYEMBRYONY (page 302), or the production of more than 
one embryo to a seed. The additional embryos have di- 
verse morphological origins, resulting oftenest from a budding 
of nucellus cells into the embryo sac the structure taking 
very perfectly the embryo form; but they grow also from 
other cells inside the embryo sacs, and from other embryo 
sacs contained in the same nucellus. The embryos them- 
selves often show a monstrosity in polycotyledony, the 
production of cotyledons in more than the normal number. 

3. The Suspension of Vitality, Resting Period, and 
Duration of Life in Seeds 

The primary seed function of serving as the disseminative 
stage of the plant involves a number of physiological features, 
of which the more prominent are indicated in the foregoing 
title. 

The value, or necessity, of a suspension of vitality 
during dissemination is qu'.te obvious, since the embryo 
plant while in transit, and hence for considerable periods of 
time, is perforce exposed to great dryness, intense light, 
destructive chemicals, etc. ; and these conditions are in- 
consistent with that continuous interchange of oxygen, 
water, and food essential to the ordinary life of plants. 
As to the actual physical method by which the suspension 
of vitality is insured in seeds, that seems to rest primarily 
upon dryness, the greater part of the water being allowed 
to escape without replacement during the ripening of the 
seed. Since water is the indispensable solvent for chemical, 
and the vehicle for physical, operations underlying growth and 
other processes, its gradual withdrawal slows the processes 
down, apparently evenly and without injury, until finally 
a point is reached at which they are barely in action, — 
precisely as engines may be slowed, by withholding of power, 



378 A TEXTBOOK OF BOTANY [Ch. VIII, 3 

to a scarcely perceptible motion. Indeed, so slow are the 
life processes in ordinary dry seeds that, as tested by the 
most important and typical process of them all, viz. res- 
piration, they are not actually demonstrable by even the 
very refined methods of research which have been applied 
to the problem. Accordingly some investigators have main- 
tained that the processes are actually suspended, as an en- 
gine may be stopped, all ready to start again when suitable 
conditions are supplied. But the collective evidence, in- 
direct as well as direct, seems rather to indicate that the 
processes never stop completely so long as the seed remains 
capable of germination. 

The extent to which the conditions of life in seeds differ 
from those of ordinary active life is attested by the extremes 
of temperature they can endure without injury. Thus well- 
dried seeds can be kept some time above the boiling point 
of water (100° C.) without damage, though active embryos 
would be killed very quickly by an exposure to only 60° C. 
Again, seeds have been kept for days surrounded by liquid 
air, at a temperature of — 194° C, and then have germinated 
freely, though active embryos would perish at 0° C. And 
seeds can endure some other untoward agencies in like man- 
ner. It is the same with the thick-walled resting spores of 
Fungi and Bacteria. 

The RESTING PERIOD, also called delayed germination, 
of seeds, is less familiar, but equally important. Some seeds 
of wild plants will germinate as soon as mature, if given 
favorable conditions of moisture and warmth; most kinds, 
however, first remain quiescent for days, weeks, months, or 
even years. Essentially the same phenomenon appears in 
the buds of trees and shrubs, for if twigs are brought into the 
warm greenhouse and placed in water, most buds will not 
start at all before February, though later, under precisely 
the same treatment, they will open and display their 
flowers to perfection. Bulbs and tubers (e.g. potatoes) act 
in a similar manner. It is true that some individual flower 



Ch. VIII, 3] VITALITY OF SEEDS 379 

buds, like some individual seeds, will start in the fall; but 
such cases are clearly abnormalities or variations, due to 
failure of the control mechanism to operate (page 342) ; and 
the result is always fatal. It is thus evident that the resting 
period is not simply an incident of seed and bud life, but is 
obligatory, so to speak, under natural conditions, though it 
can be shortened artificially in a good many cases. The 
functional value, or necessity, of the resting period is obvious, 
since it tends to prevent the germination of seeds and open- 
ing of buds in warm times of late autumn or winter, when sub- 
sequent freezing must inevitably kill the new growth. As to 
the physical basis of the resting period (the method by which 
it is enforced on the seed), that seems to be diverse. In 
some cases it is known to depend upon the embryo, con- 
sisting in a slow '' after-ripening," i.e. formation of enzymes, 
acids, or other essential substances; but in other cases it 
has been proven to depend upon the character of the seed 
coats, which are so constructed as to prevent the admission 
of oxygen or of water, both indispensable to germination, — 
the inhibition continuing until the coats are ruptured by de- 
cay. It is of course a necessary corollary of this explanation 
that in such cases germination will be prompt if the seed 
coats are artificially broken ; and such is found by experiment 
to be true and has long been known to nurserymen and 
gardeners. Thus, they break Peach pits with a hammer, open 
Canna seeds with a file, and bruise or break the coats of 
others in diverse ways, thereby greatly hastening the germi- 
nation of those kinds. 

While the seeds of most plants have a resting period, 
cultivated plants seem mostly to lack it. Thus, we grow 
Corn, Beans, Peas, and other crop plants in our laboratories 
in autumn from seeds of that summer. This pecuharity, 
indeed, sometimes brings loss to the farmer, since in excep- 
tionally warm wet autumns, grain is apt to germinate in the 
ear in the standing crop, to its very great damage. The 
resting period has presumably been lost from cultivated 



380 A TEXTBOOK OF BOTANY [Ch. VIII, 3 

plants through its complete disuse during the many cen- 
turies of their cultivation by man, who has attended to the 
safety of the crop himself and directed his selection to quite 
other qualities. 

The DURATION OF LIFE, or VIABILITY, in seeds is most 
various. Every one who works with a garden knows that 
some kinds keep good for only one season, while others last 
two or three ; and methods exist for testing the viability in 
cases of doubt. There are kinds which must germinate the 
summer they are formed, or not at all ; and this is true of 
Elm, Willow, and Poplar, — trees which form their seed 
early in spring. Most kinds, however, wild as well as cul- 
tivated, if kept dry and cool, remain viable for one, or two, 
perhaps three years, though beyond that period the number 
of kinds which survive steadily wanes with advancing years. 
Tests made on seeds taken from dated museum or herba- 
rium collections have shown indubitable germination in seeds 
eighty-seven years old, with a possible case over one hun- 
dred and twenty years. It is interesting to note, by the 
way, that these extreme longevities occur in seeds possessing 
thick hard coats. As to the reported germination of seeds 
taken from the wrapping of mummies, or from ancient tombs, 
hundreds or thousands of years old, it is not confirmed by 
the exact methods of science, while on the other hand there 
is ample evidence that seeds are often introduced fraudu- 
lently into such places. 

What then actually ends the viability of such seeds? If 
they can live so long in the inert state, why not indefinitely? 
The very fact, by the way, that all die, and mostly within a 
few years, is presumptive evidence for the view that the life 
processes are not in suspension, but only slowed down. The 
death of the seed comes gradually, and without any visible 
external sign, in most cases at least; and it clearly is not 
due to exhaustion of food or like kind of cause. Here, how- 
ever, our knowledge ends. Possibly the loss of water can 
proceed to a fatal degree ; perhaps the accumulation of waste 



1 



Ch. VIII, 4] GERMINATION OF SEEDS 381 

products of the slow metabolism within the tightly-sealed 
seed coats poisons the embryo ; and it may be that the slow 
coagulation of the proteins destroys the essential constitu- 
tion of the protoplasm. Between these possibilities, and 
perhaps others, the future will decide. 

4. The Germination of Seeds 

The seed, its resting period completed, germinates on 
access of water, air, and warmth. The water it needs to 
expand its parts; the air is necessary for its respiration, 
which is very active in all growth ; the warmth is required 
to accelerate the many physical and chemical processes in- 
volved. As to light, that has no influence, direct or indirect, 
in most cases, though special seeds are known which will not 
germinate in light, and others which will not germinate with- 
out it, doubtless for reasons incidental to some peculiarity 
of their metabolism. 

In germination we can distinguish some seven stages. 
First, most seeds, though not all, swell greatly throughout, 
often to more than double their dry size, by absorption 
of water, which enters partly by imbibition and partly by 
osmosis. As these words imply, the absorption is forcible, 
and thus seeds can lift considerable weights in the ground or 
break strong containers under experiment. 

Second, the seed coats are broken, no matter how thick 
and strong, by the pressure from within. In some the 
rupture is irregular ; in others, it follows definite lines cor- 
responding with angles or depressions of the coats. Some 
very striking special arrangements to this end are known 
(Fig. 273). 

Third, the digestion of the food substances begins. The 
insoluble starches, oils, and proteins are converted by en- 
zymes into soluble sugars, fatty acids, and peptones, as 
manifest to the eye in the change from opacity to trans- 
lucency, and a softening of the seed. Then the digested food, 
absorbed by the cotyledons in albuminous seeds, though 



382 



A TEXTBOOK OF BOTANY [Ch. VIII, 4 



already within them in ex-albuminous kinds, is ready for 
translocation, and use in the growing parts of the embryo. 
Fourth, the end of the hypocotyl of the embryo, lying next 
the micropyle, now pushes forth, and as soon as clear of the 
seed coats, grows geotropically over to point downward, 
developing meantime the root at its tip. This root is a 
new growth, and not a transformation of the hypocotyl, as 
students are prone to suppose. Then, if the seed, as is 
usual with wild plants, lies on the sur- 
face of the ground, the root begins to 
enter the earth. No sooner does the root 
start into the soil than (from small seeds 
at least) it sends out a radiating ring or 
collar of root hairs which take firm hold 
on the rock particles. Thus is provided 
a resistance, without which further 
growth might rather lift the seed from 
the ground than force the root into the 
soil. In some other seeds, such as Flax, 
such resistance is provided by a muci- 
laginous coat which gums it, so to speak, 
to the ground. Practically all embryos, 
as the first act of their development, 
thus secure access to the water supply 
which is indispensable to their further development. 

Fifth, on the basis of the anchorage secured by the pene- 
tration of the root into the earth, the hypocotyl now begins 
to make such growth movements, too complex for easy de- 
scription but readily shown in our pictures (Fig. 274), as 
cause the withdrawal of the cotyledons from the seed coats, 
and their subsequent elevation, when they open out to the 
light. In cases, however, like Peas and some Beans, where 
the cotyledons are apparently too thick to serve later as 
effective foliage leaves, they remain in the ground, while 
the plumule issues from between them, and grows geotropi- 
cally upward. 




Fig. 273. — Germi- 
nating seed of Pump- 
kin, showing the "peg" 
or "heel" by the devel- 
opment of which the 
seed coat is forced open. 
(From F. Darwin.) 



Ch. VIII, 4] GERMINATION OF SEEDS 



383 




384 A TEXTBOOK OF BOTANY [Ch. VIII, 4 

Sixth, the parts which rise in the Kght, especially the 
cotyledons and plumule, as they issue from the seed coats, 
begin to turn green, and, by the time they are spread open 
at the top of the young stem, have their full quota of chloro- 
phyll, in obvious preparation for the manufacture of new- 
food. 

Seventh, the enlargement of hypocotyl, cotyledons, and 
plumule proceeds by absorption of water until all of the 
cells laid down in the embryo are fully expanded, at which 
time, with the root firmly fixed in the ground, the young 
stem is erect with the cotyledons fully green and expanded. 
Germination is now complete, and the germinated embryo 
is ready to continue development, with formation of new 
parts, into a seedling. It is true, the formation of new 
leaves and buds does not always await the completion of 
the expansion of embryonic parts, but in principle at least 
there is this distinction between germination and the sub- 
sequent growth of the seedling. 

If a fully germinated embryo be compared point by 
point with one from a resting seed, as may best be done with 
some of the compact succulent kinds like Cactus, the fol- 
lowing differences appear. First, except for the root and the 
chlorophyll, the germinated embryo possesses nothing really 
new. Second, it has become many times larger, even to 
twenty or thirty times. Third, again excepting the root, it 
has usually few new cells, the enlargement having consisted 
chiefly in the increase in size of those already developed. 
Fourth, the cells are now all apparently empty (except for 
water) instead of densely packed with solid food, thus 
explaining the watery translucency of the germinated 
embryo as contrasted with the white opacity of its unger- 
minated condition. Fifth, its dry weight, determined by 
comparative weighings of oven-dried material, is actually 
less, showing that the far greater bulk consists chiefly of 
water. Thus it is clear that germination consists primarily 
in the great expansion through water absorption of the 



Ch. VIII, 5] ECONOMICS OF SEEDS 385 

close-packed cells of the original embryo, the food being 
used partly in the formation of the root and partly in the 
enlargement of cell walls. Evidently the functional point 
of the process is found in the great spread of green surface 
thus quickly achieved by the use of a relatively small amount 
of solid material. The value of the spread of surface in this 
case is obvious, for the young plant has to begin as early as 
possible the acquisition of its own photosynthetic food supply. 

5. The Economics and Cultivation of Seeds 

Among all of the parts of plants, seeds stand preeminent 
in direct utility to man. This of course is because they 
include the grains, Corn, Wheat, Rice, Barley, Rye, and some 
others, together with the leguminous crops. Beans, Peas, 
Millet, which collectively make up the greater part of the 
food supply of mankind. These seeds contain rich stores of 
starches, oils, and proteins, originally laid down by plants 
for the use of their embryos, and now taken for his needs by 
man, who has been able through long centuries of cultivation 
and breeding to greatly increase their yield both in quantity 
and quality. Of a different kind is one other great economic 
use of seeds, viz., the fibrous hairs developed by the Cotton 
seed as its disseminative mechanism (by wind) yield the 
cotton of commerce (Fig. 254). 

The grains, as earher noted (page 349), are fruits as well 
as seeds, the seed coat and ovary wall being grown together 
into one structure which constitutes the husk. The husks 
are removed in milling white flour, but retained in graham 
flour, which is the more nutritious because it includes the 
layer of protein-storing cells which form the outermost part 
of the food in the grain (Fig. 65). 

The agricultural and horticultural treatment of seeds 
appears to offer nothing peculiar, the various principles of 
cultivation and breeding being the same as with other parts. 
There is, however, one economic matter peculiar to seeds, 
in connection with their viabiHty. Since nothing in the 
2c 



386 A TEXTBOOK OF BOTANY [Ch. VIII, 6 

aspect of a seed tells whether it is still alive or not, or what 
percentage of a given quantity is ahve, the purchaser .of seeds 
is at the mercy of a dealer unless he can himself make test of 
viabihty. For such tests various methods have been devised, 
the most simple and direct of which is that of placing a 
given number in folds of blotting paper kept wet, dark, and 
well aerated, and noting the percentage which germinates. 

6. The Cycle of Development from Seed to Seed 

Having studied the six primary parts of plants with respect 
to their structures and functions, it remains to consider 
their successive appearance in that cycle of development 
through which every individual passes. It is possible to 
break the cycle for study at any desired point, but in prac- 
tice we may best start with the germinating seed. The facts 
having already been considered in detail, we can best review 
the subject in a way to bring out its general principles. 

The seed contains a well-formed embryo plant, provided 
with stem, rudiments of root and bud, and cotyledonary 
leaves, all enwrapped with a store of food substance inside 
protective coats. In germination the seed absorbs water, 
swells, and bursts the coats ; the stem pushes forth its 
lower end, which grows over geotropically downward 
and enters the ground. Meantime its tip is developing a 
root, which, on contact with the soil, puts forth many root 
hairs, whereby it absorbs osmotically a sufficiency of water. 
No sooner is the root secure in the ground than the stem 
makes growth bendings which first withdraw the cotyledons 
from the seed coats, and then lift them geotropically upward 
until they open out to the fight on the tip of the vertically 
straightened stem. Meantime the whole plant is swelfing 
rapidly in size through absorption of water, and turning 
green over stem and leaves by formation of the chlorophyll 
so essential to its future welfare. Thus the fully germinated 
EMBRYO now stands rooted in the ground and erect in the 
sun, to which it spreads a large surface of chlorophyll. In 



Ch. VIII, 6] CYCLE OF DEVELOPMENT 387 

this process all of the food supplied by the parent plant has 
been used ; and thenceforth the new plant must depend en- 
tirely upon its own physiological powers, for the exercise 
of which, however, it is now fully prepared. 

The successive stages in the developmental cycle of plants, 
while distinct in principle, largely overlap in practice, so 
that even before the completion of germination, the young 
plant has commenced the activities of its next, or seedling, 
stage. With the spread of its chlorophyll in hght, it begins 
to acquire a new food supply of its own, which forms a 
basis for further development. The root now begins to send 
out branches, diageotropically guided either horizontally or 
at definite angles from the vertical main root, though these 
directions of growth are soon disarranged by obstructions in 
the soil. Meantime the plumule bud, between the cotyledons, 
is continuing its development, forming in symmetrical order 
new leaves, which, at first small and tightly appressed to 
the stem, later gradually open out until they present their 
full faces to the sun. Simultaneously there is continuous 
increase in size, and the formation of suitable firm support- 
ing and other needed tissues. Thus is attained the stage of 

the SEEDLING. 

Gradually the seedling passes into a stage which in case 
of trees is called the sapling. In the roots new branches 
spring from the secondary roots, not at definite places or 
angles, but guided hydrotropically and chemotropically 
towards the moistest and richest parts of the soil, where 
they develop more profusely, thus making the root system as 
asymmetrical as the soil is irregular in texture. Meantime, 
while the leaves are still in the embryonic stage, new buds 
develop in their axils, and later, after those leaves have 
passed their maturity and fallen, grow out into branches 
which bear new leaves in precisely the same manner as does 
the main stem. These branches, guided diageotropically, 
grow out at definite angles with the vertical main trunk, 
and, possessing also the same symmetrical phyllotactic ar- 



388 A TEXTBOOK OF BOTANY [Ch. VIII, 6 

rangement as the leaves, tend to build stem-and-leaf structures 
very symmetrical in plan. Meantime also the special tissues 
which give strength and meet other needs are continuing to 
develop in places required by stress or other demand. 

In this stage appears the striking seasonal cycle imposed 
on all plants outside of the tropics by the extreme alter- 
nation between summer and winter. The summer alone has 
the warmth to permit full vital activity in plants, and ac- 
cordingly is the season of green vegetation, accumulation of 
food, and development of new parts. In the autumn prep- 
aration is made for the winter, and accordingly that is the 
season when fruits are ripened, buds are enwrapped in their 
scales, leaves are cut off and dropped, and tissues are par- 
tially dried; while the attractive colors of fruits and the 
varied hues of dying leaves make it a time of bright color 
in vegetation. The winter is the season of enforced dor- 
mance, when the dried tissues of plants, approaching the 
conditions in seeds, remain almost inactive within their 
nearly sealed wrappings, which display no colors other than 
their incidental grays or browns. The spring is the season 
of unfolding, when the ready-formed parts, amply supplied 
with stored food, absorb copious water, enlarge, burst their 
wrappings, and push forth green leaves to make new food, 
and bright flowers to effect fertilization ; and all vegetation 
wears the soft colors of the new-forming tissues. This is 
the four-part seasonal cycle through which our perennial 
plants pass every year as long as they live. 

The next stage of the developmental cycle is the adult. 
It is not distinguished from the sapling by attainment of any 
fixed size, for plants (unlike animals) continue to grow, by 
formation of new parts, as long as they Hve. Nor is it 
marked by any change in the mode of formation of roots, 
buds, or leaves, which continue to be made in the same gen- 
eral way. It is true, a gradual loss of the youthful sym- 
metry accompanies advancing age in trees and shrubs, 
partly because of the interference of the over-plentiful 



Ch. VIII, 6] CYCLE OF DEVELOPMENT 389 

branches with one another, partly because of accidents, and 
partly because of photo tropic and other self-adjustments. 
The real mark of adult age is the beginning of sexual repro- 
duction. After the young plant has attained a considerable 
growth, presumably accumulating food in reserve, some 
of the axillary buds, precisely ahke in position and mode of 
formation to those which have been producing leafy branches, 
begin to produce flowers, — that is, speciahzed determinate 
branches containing reproductive spores which develop the 
sexual cells. As to the nature of the stimulus which leads 
the plant thus suddenly to convert certain of its branch 
buds into flower buds, or more exactly, to develop reproduc- 
tive spores with the correlated floral structures, we have as 
yet no exact knowledge, although the influence of various 
external factors is clearly apparent. Having once begun to 
produce the flowers, the plant continues to make them, just 
as it makes leaves, branches, and roots, as long as it hves. 
The central parts of these flowers are pollen grains and em- 
bryo sacs, which in turn develop the two kinds of sex cells. 

The next stage in the cycle includes fertilization. The 
floral parts are essentially organs functionally fltted to effect 
union of the sex cells, — and a union usually between two 
different parental strains. By utihzation of the motive 
power of winds, insects, etc., the pollen containing the sperm 
cell is transported from its place of formation to the vicinity 
of the deeply-buried egg cell, after which the growth of a 
pollen tube brings egg cell and sperm cell together into a 

single FERTILIZED EGG CELL. 

The next stage is that of the development of the fertilized 
egg cell into an embryo. The s"ngle cell, lying in the 
embryo sac, begins at once to divide and to grow, then 
divides again and grows farther, and thus, under guidance 
of influences partly hereditary and partly environmental, 
it gradually assumes the form of the many-celled embryo, 
with its stem and cotyledons. Meantime the endosperm or 
food substance is forming around the embryo, and the hard 



390 A TEXTBOOK OF BOTANY ICh. VIII, 6 

seed coats are developing around both. Thus is reached the 
stage of the fully formed embryo within the seed. 

The final stage is that of dissemination, performed by the 
SEED. A considerable time often elapsing either before 
transport or during that process, with simultaneous 
exposure to extreme conditions, the seed goes into a resting 
condition with all of its processes reduced to a minimum, 
and with provision against premature germination. Then, 
separating from the parent plant, it becomes transported 
by wind, animals, or other locomotive agency, acting upon suit- 
ably developed mechanisms, to a distance sufficient to per- 
mit the free development of its plant without interference 
with the parent. Having attained a suitable place, its 
resting period ended, and water, air, and warmth sup- 
plied, the seed germinates. But with germination the 
cycle is closed. If the term cycle seem inappropriate, since 
the return is not to the same seed, then the simile of the 
spiral, winding back to the same starting line, may better 
express the process. 



INDEX 



Figures in heavy type indicate pages on which illustrations occur. 



Abnormalities, 196. 

Abrus, 363. 

Absciss-layer, 120. 

Absorption, 262 ; by roots, 224. 

Adaptation, 12. 

Adhesive seeds, 362. 

Adult, 388. 

Aeration system, 132, 266. 

Aerenchyma, 252, 266. 

Aerial roots, 254, 256. 

Aerotropism, 232, 248. 

Estivation, 329. 

After-ripening, 379. 

Agriculture, 4. 

Air, in soils, 240. 

Air-passages, 19, 29. 

Air system, 33. 

Akene, 348, 349. 

Alcohol, production, 172 ; source, 

101. 
Algffi, 12 ; Red, 305. 
Alkaloids, 109. 

Alternation of generations, 301. 
Alveolar, 37. 
Anatomy, 3, 8. 
Anchorage by roots, 231. 
Animals, nutrition, 86 ; seed carriage, 

361. 
Annual rings, 124. 
Annuals, 114. 
Anoxyscope, 167, 168. 
Anther, 272. 
Antheridium, 306. 

Anthocyanin, 88 ; composition, 108. 
Antitoxin, 173. 
Ants, in dissemination, 363. 
Apogeotropic, 247. 
Appendages, 376. 
Archegonium, 306. 
Areas of chlorenchyma, 32. 
Aril, 351, 375. 
Aristolochia, anatomy, 129. 
Asexual spores, 301. 



Asexual ?;s. sexual propagation, 302. 
Asparagus, "fasciated, 197. 
Automatism, 39. 
Autumn coloration, 88 ; effect of 

external conditions, 93. 
Auxograph, 155, 156 ; record, 156. 

Bacteria, 84, 244, 368; nitrifying, 
244 ; nutrition, 84 ; in soils, 244. 

Bacteriology, 4. 

Bailey, L. H., Cyclopedia, 60. 

Bald Cypress, 252. 

Balfour, Class Book, 58. 

Bamboo, 127, 179. 

Banana, 58. 

Banyan, 253, 254. 

Bark, abscission, 123. 

Barton, Botany, 75. 

Bast, 130; fibers, 131, 265; paren- 
chyma, 131. 

Beet rings, 256. 

Begonia phyllomaniaca, 204. 

Berry, 350. 

Biennials, 114. 

Bignonia seed, 358. 

Bird's-eye Maple, 198. 

Birds, as cross-pollinators, 294 ; in 
dissemination, 363. 

Black Knots, 367. 

Bleeding, 151, 227. 

Blights, 367. 

Blood heat, 169. 

Blotch diseases, 367. 

Bordeaux mixture, 369. 

Botany, definition, 1 ; study, 2 ; 
subdivisions, 2. 

Bracket, on stems, 182. 

Bract, 73, 271, 276; colored, 74; of 
Linden, 74 ; in Poinsettia, 74. 

Branch, 183. 

Bryophyllum, 71 ; 299. 

Bryophytes, 11 ; low growth of, 144. 

Bud, accessory, 137 ; adventitious, 



391 



392 



INDEX 



137 ; anatomy, 138 ; axillary, 137 ; 
defined, 135 ; on leaves, 71 ; of 
Palm, 136; scales, 78, 80; sepa- 
rable, 300 ; sizes, 136 ; sport, 209 ; 
terminal, 137 ; unregulated de- 
velopment, 198 ; winter, 135. 

Bulb, 73, 300 ; forms, 74. 

Bulblet, 373. 

Bundle-sheath, 30. 

Burbank, 321. 

Burdock head, 362. 

Burls, 199, 200. 

Bursting pods, 365. 

Btitschli, 38. 

Button Bush, 336. 

Cabinet woods, 205, 

Cactus, 189. 

Caffein, 109. 

Caloriscope, 170. 

Calyx, 269, 351. 

Cambium, 118; described, 132; 

growth from, 124. 
Camphor, 108. 
Cankers, 367. 
Caoutchouc, 108. 
Capillarity, 148, 237. 
Carbohydrates, 100 ; value, 100. 
Carbon dioxide, absorption by plants, 

22. 
Carotin, 90 ; composition, 108. 
Carpel, 273, 351. 
Caruncle, 363, 375. 
Cavers, Botany, 147. 
Cedar apples, 347. 
Cell, contents, 42 ; definition, 8 ; 

division, 281, 283; initial, 355; 

sap, 30 ; shapes, 42 ; structure, 

41 ; wall, thickened, 103. 
Cellulose, 41 ; composition, 98 ; 

uses, 99. 
Cement, in trees, 211. 
Central cylinder, 264. 
Chalaza, 274. 
Chalazal angle, 376. 
Chemosynthesis, 87. 
Chemotropism, 249. 
Chestnut disease, 356. 
Chimsera, 210, 366. 
Chlorenchyma, 17, 29, 262; areas, 

32 ; thickness, 53. 
Chlorophyll, 17, 108, 386; composi- 
tion, 108 ; function, 25 ; spread, 387. 



Chloroplastids, 30. 

Chlorosis, 368. 

Chondriosomes, 41. 

Chromatin, 280. 

Chromosome mechanism of heredity, 

310. 
Chromosomes, 280 ; diagram, 282 ; 

significance, 282. 
Chrysanthemum, 318. 
Cion, 208. 
Cladophylla, 196. 
Clambering stems, 184. 
Classification, 2, 10. 
Cleistogamous flowers, 290. 
Cleistogamy, 292. 
Clematis fruit, 350. 
Climbers, 9, 184. 
Clinostat, 174, 176. 
Close-pollination, 287. 
Cluster, 268 ; of flowers, 335. 
Cocaine, 109. 
Cocklebur fruit, 363. 
Coconut, 345, 361, 362, 372. 
Collenchyma, 118, 265. 
Colors of leaves, 88, 90 ; brown, 92 ; 

green, 88 ; non-green, 88 ; red, 88 ; 

white, 90 ; yellow, 89. 
Columbine, 295 ; pods, 347. 
Companion cell, 131. 
Compass plants, 58. 
Conduction, 262 ; of carbohydrates, 

152 ; of proteins, 152. 
Cone, 352 ; 353. 
Constriction of stems, 152. 
Conventional constant, 25. 
Convolute, 329. 
Copper Beech, 319. 
Cordage, 206. 
Cordyline, 65, 

Cork, 261 ; cambium, 264 ; de- 
scribed, 133 ; uses, 99. 
Corm, 191. 

Corn bundle, 135 ; stem, 119. 
Corolla, 270. 
Corona, 82, 332, 333. 
Cortex, of roots, 220. 
Cortical system, 262. 
Corymb, 336, 337. 
Cotton, 205 ; seed, 359. 
Cotyledons, 73, 355, 374. 
Crested forms, 197. 
Cross-pollination, 286, 287 ; meaning, 

298. 



INDEX 



393 



Crown Galls, 367. 
Cryptogams, 12. 
Crystals, 33; in plants, 111. 
Curly Birch, 200. 
Curly Top, 369. 
Cutin, 32, 98. 
Cuttings, 259. 
Cyme, 334. 
Cypripedium, 292. 
Cytisus Adami, 210. 
Cytology, 3. 
Cytoplasm, 41. 

Daffodil, 333. 

Dandelion fruit, 360. 

Darwin, 316. 

Darwin, F., 120. 

Decay, nature, 172. 

Dehiscence, 346, 347. 

Dermal system, 262. 

Dermatogen, 264. 

Desert vegetation, 48. 

Determiner, 309. 

Development, 153 ; cycle, 9, 386 ; 

described, 154. 
De Vries, 316. 
Dextrose, 100. 
Diageotropic, 247. 
Dichogamous flower, 288. 
Dichogamy, 291. 
Differential thermostat, 157. 
Diffusion, 236 ; described, 236. 
Dimorphic flowers, 289. 
Dimorphism, 292. 
Dioecious plants, 307. 
Disbudding, 207. 
Diseases of plants, nature, 367. 
Dispersal, 356. 
Dissemination, 266, 356. 
Distillation, 172. 
Division, 39, 299. 
Dodder, 83, 84, 256. 
Dodel-Port, 277. 
Dominant, 311. 
Double fertilization, 354. 
Dragon tree, 115, 127, 128. 
Drainage, 167, 
Drip point, 68, 69. 
Drupe, 350, 351. 
Dry farming, 261. 
Duct, 31, 122, 130, 146, 262 ; length, 

146. 
Duggar, Physiology, 123. 



Durian, 346. 

Dust, on plants, 96. 

Ecology, 4. 

Economic botany, 4. 

Egg, 280. 

Egg cell, 9, 273, 274, 278, 304 ; fer- 
tilized, 389. 

Elementary species, 317. 

Elements essential to plants, 230. 

Elm fruit, 358. 

Embryo, 9, 373, 374; development, 
355; germinated, 384, 386; plant, 
386 ; sac, 274. 

Embryology, 3. 

Emergences, 33. 

Endodermis, 222, 262. 

Endogenous, 127 ; growth, 128. 

Endosperm, 374. 

Energy, kinetic, 166 ; potential, 166. 

Enlargement, 153 ; described, 154. 

Enzyme, 85; description, 110. 

Epidermal cells, 32. 

Epidermis, 18, 29, 261 ; cells, 32. 

Epiphyte, 9, 185 ; funnel form, 
185. 

Epiphytic, Fern, 186 ; Orchid, 184. 

Erica leaf, 70. 

Errera and Laurent, 160. 

Erythrophyll, 88 ; formation, 91 ; 
functions, 88. 

Essences, 108. 

Essential oils, 107. 

Evolution, 13, 308, 315. 

Excretion, 266. 

Exogenous, 126. 

Extension through growth, 357. 

Fall plowing, 260. 

Fallow, 260. 

Fasciated Asparagus, 197 ; Echino- 

cactus, 198 ; Pineapple, 198. 
Fasciation, 197, 367. 
Fatty oil, 104 ; as food, 104 ; kinds, 

104. 
Fermentation, 169 ; demonstration, 

171 ; equation, 171. 
Fern, plants, 1 1 ; reproduction, 306 ; 

seed, 373. 
FertiUzation, 277, 279 ; double, 354 ; 

significance, 286. 
Fertilizers, role, 242 ; use, 260. 
Fibonacci series, 142. 






394 



INDEX 



Fibro-vascular, bundles, 116, 118; 
system, 118. 

Fig, 352. 

Figurier, Vegetable World, 73. 

Filament, 272. 

Films, of water, 239. 

Fir tree, 180. 

Fleshy fruits, dissemination, 362. 

Flora, 3. 

Floral diagrams, horizontal, 326, 328, 
329 ; numerical plans, 328 ; verti- 
cal, 331 ; the whorls, 326. 

Flower, cleistogamous, 290 ; colors, 
267 ; complete, 276 ; dichogamous, 
288; dimorphic, 289, 292; dura- 
tion, 269 ; disk, 339 ; economics, 
343 ; features, 267 ; function, 8 ; 
geotropism, 297 ; hermaphrodite, 
307 ; insect pollinated, 290 ; ir- 
regular, 276, 293 ; monstrosities, 
340; morphology, 183, 322; neu- 
tral, 338, 339; odors, 268; per- 
fect, 276, 307 ; phototropism, 
296; pistillate, 276, 285; polli- 
nated by bee, 291 ; preservation, 
344; ray, 338; regular, 276; 
staminate, 276, 285 ; structure, 
269 ; typical, 270 ; wind-pollinated, 
288. 

Flowering plants, 10. 

Fluctuations, 314. 

Fodder, constituents, 101 ; plants, 
206. 

Foliage, autumnal coloration, 90 ; 
support, 265 ; variegated, 90. 

Follicle, 348. 

Food, 28, 374; of animals, 112; 
reserve, 100 ; synthesis, 19. 

Forestry, 4, 205. 

Freaks, 72, 196. 

Frost plant, 52. 

Fructification, 347. 

Fructose, 100. 

Fruit, acids, 110; aggregate, 352; 
defined, 345 ; dehiscence, 346 ; 
dots, 324, 373 ; dry, 345 ; econom- 
ics, 370 ; features, 345 ; forma- 
tion stimulus, 352 ; fleshy, 345 ; 
function, 8 ; monstrosities, 366 ; 
morphology, 347 ; multiple, 352 ; 
relation to ovary, 345 ; simple, 352 ; 
spurs, 183 ; twin, 196, 199 ; two- 
storied, 367. 



Fuchsia, 332. 

Fucus, 189. 

Fungi, 11, 84; colors, 8( 
by, 368 ; nutrition, 84 
367 ; in soils, 244. 

Fusion nucleus, 353. 

Fusion of germ cells, 280. 



; damage 
parasitic, 



Galls, described, 203 ; typical, 204. 

Gamete, 287, 303. 

Gamopetalous, 271. 

Gamosepalous, 270, 330. 

Gelatination, 99. 

Gemmae, 300, 

Generation, skipping a, 311. 

Genetic variations, 314. 

Genetics, 4. 

Genotypically, 310. 

Geotropism, 174, 175, 296; function 
of, 177; lateral, 255; of Mush- 
rooms, 178 ; of roots, 174 ; of 
stems, 175. 

Gerardia, Purple, 87. 

Germ cell, 280; fusion, 280; purity 
of, 311. 

Germination, 381 ; delayed, 378 ; 
of Lima Bean, 383 ; movements, 
382 ; of mummy seeds, 380 ; 
of pollen, 275; stages, 381. 

Giant Kelp, 190. 

Gland, ethereal oil, 107. 

Globuhn, 105. 

Glucose, 100. 

Glucoside, 110. 

Glutelin, 105. 

Gnarls, 199. 

Goebel, Schilderungen, 62. 

Gourd, 351. 

Graft-hybrids, 210. 

Graftage, 208. 

Grafting, 208, 209, 371 ; results, 210. 

Graham flour, 885. 

Grain, 317, 385; Corn, 375; im- 
portance, 385 ; structure, 349. 

Grand period, 156, 157 ; described, 
156. 

Grape sugar, formula, 21 ; role in 
plant, 27. 

Gravitation, effects on plants, 175. 

Gray, Botany, 16. 

Greenhouse construction, 95. 

Green-manuring, 260. 

Greenness of vegetation, 26. 



INDEX 



395 



Growth, 39, 264 ; definite annual, 
138 ; described, 153 ; control 
mechanism, 342 ; effect of humidity 
on, 158 ; effect of light on, 158, 
159 ; effect of temperature on, 
157, 158 ; of general tissue, 
354 ; grand period in roots, 221 ; 
indefinite annual, 138 ; of leaves, 
161; primary, 119; of roots, 
161; secondary, 119; of stems, 
160. 

Guard cells, 33, 49, 50, 262; oper- 
ation of, 49. 

Gum, 104. 

Gum tree, 113. 

Guttation, 52. 

Gymnosperm, 352. 

Haberlandt, Anatomy, 31. 

Hair-like structures, 351. 

Hairs, 70. 

Half parasite, 87. 

Haustorium, 83, 256. 

Head, 336, 337. 

Healing of injuries, 123, 206. 

Health in plants, 369. 

Heart wood, 124, 145. 

Heat of respiration, 168. 

Heliotropism, 54. 

Hemi-cellulose, 103. 

Herb, 9. 

Herbarium, 3. 

Heredity, 10, 13, 39, 128, 285, 308 ; 

defined, 308, 314. 
Heterozygous, 310. 
Hilum, 376. 
Histology, 3. 
Hollow column, 180. 
Homozygous, 310. 
Honesty, 348. 
Hooks, 361. 

Horse Chestnut twig, 120. 
Horticulture, 4. 
Host, 83. 
Hotbeds, 258. 
Houseleek, 142. 
House plants, 48, 241. 
Humus, described, 241, 243. 
Huxley, 35. 
Hybrid, 320. 

Hybridization, 318 ; method, 320. 
Hydrangea, 339. 
Hydrophyte, 190. 



Hydrotropism, 177 ; described, 247. 
Hygroscopic phenomena, 237 ; tis- 
sues, 366. 
Hypocotyl, 355, 374. 

Idioblasts, 33. 
Imbibition, 148, 237. 
Imbricate, 329. 
Immune varieties, 370. 
Improvement of plants, 2. 
Indian Pipe, 83, 85. 
Inhibitory influence, 202. 
Initial cell, 355. 
Injuries, healing, 122. 
Insect-pollinated flowers, 290 ; char- 
acteristics, 290. 
Insects as cross-pollinators, 290. 
Integuments, 274. 
Intercellular air system, 33, 266. 
Internode, 116. 
Involucre, 339. 
Iodine test, 20. 
Iris flower, 287. 
Ironwood, 113. 
Irritability, 39, 55. 
Ivory Palm, 373. 

Jack fruit, 346. 
Jussicea, 252. 

Kerner, Pflanzenleben, 57. 
Knees, 252. 

Knowledge, 5 ; useful, 5. 
Kny, L., 133. 

Laciniate, 203. 

Lamarck, 315. 

Latex, 108, 109; system, 134; sys- 
tem, described, 134. 

Lathyrus Aphaca, 78, 80 ; pod, 
365. 

Le Maout and Decaisne, Traite, 76. 

Leaf, anatomy, 28, 29 ; arrange- 
ments, 139 ; auriculate, 68, 69 ; 
axil of, 73 ; of Bidens Beckii, 62 ; 
characteristics, 15 ; compound, 16, 
67 ; connate-perfoliate, 69 ; eco- 
nomics, 94 ; entire, 68 ; as a 
factory, 26 ; functions, 7, 72 ; 
linear, 63, 63 ; lobed, 67 ; margins, 
68 ; morphological plasticity, 82 ; 
mosaic, 56 ; netted-veined, 17, 
66; orbicular, 62, 63; ovate, 64, 



396 



INDEX 



66 ; palmately compound, 68 ; 
parallel- veined, 17, 66 ; perfoliate, 
68, 69 ; pinnately compound, 68 ; 
pitchered, 202, 203; plan, 34; 
scars, 120 ; serrate, 68 ; shapes, 
62, 68 ; simple, 16 ; storage 
function, 72 ; structure, 17 ; ten- 
drils, 76, 78 ; thickness, 16 ; trace, 
119; typical, 16 ; venation, 18. 

Leaflet, 16. 

Leaves, arrangements, 139 ; alternate, 
140, 141, 142 ; opposite, 139 ; varie- 
gated, 89 ; whorled, 140. 

Legume, 348. 

Leguminosse, vs. Bacteria, 246. 

Lenticel, 120, 121; described, 121. 

Lettuce bud, 136. 

Life history, 3. 

Light, adjustment, 52 ; leaves ad- 
justed, 57 ; role in plant, 26 ; 
screen, 20. 

Lignin, 98. 

Linden, bract, 74 ; bundles, 133. 

Linen, 205. 

Linnsean species, 317. 

Linnaeus, 7, 315. 

Lipase, 110. 

Liverworts, 11. 

Loam, 241. 

Locomotion, 357. 

Long Moss, 185. 

Lumber, 205. 

Mangrove, 253. 

Manual, 3. 

Maple fruit, 351. 

Marcgravia, 294. 

Martynia, 362, 363. 

Masters, Teratology, 201. 

Maturation, 153. 

Mechanical, effects, 196 ; system, 
265. 

Mechanistic, conception of nature, 
39, 40. 

Medullary ray, 122, 265; descrip- 
tion, 125 ; secondary, l25. 

Megasporangium, 324. 

Megaspore, 324, 

Mendel, 309. 

Mendel's Law, 312, 313. 

Meristem, 128, 264. 

Mesembryanthemum, 72. 

Mesophyte, 190. 



Metabolism, 39, 98, 266 

Microorganisms, 243. 

Micropyle, 376. 

Microscope, 28. 

Microsporangium, 325. 

Middle lamella, 147. 

Mildews, 367. 

Milkweed seed, 359. 

Milky juice, 134. 

Mineral salts, 230, 242 ; use, 28. 

Mistletoe, 86, 187, 362. 

Mitochondria, 41. 

Mobihty, 39. 

Molds, 11. 

Monadelphous, 272. 

Monocarpic plants, 114. 

Monocotyledons, 127. 

Monoecious plants, 307. 

Monstrosities, 72, 196; cause, 342; 
of flowers, 340 ; of stems and leaves, 
196. 

Morphine, 109. 

Morphological, diagram, 353 ; plas- 
ticity, 39. 

Morphology, 3 ; definition, 82. 

Mosaic disease, 369. 

Moss, flowers, 269; plants, 11. 

Muck, 241. 

Mulberry, 352. 

Multiple fruit, 352, 363. 

Mutation, 13, 314, 317. 

Mycelium, 84. 

Mychoriza, 83, 244. 

Natural selection, 316. 

Navel Orange, 201, 205, 319, 367. 

Nectar, 343. 

Nectary, 275 ; forms, 273. 

Nelumhium, 361. 

Nemalion, 305. 

Nepenthes, 76, 246. 

Nicotine, 109. 

Nitrates, 242. 

Nitrogen fixation, 244. 

Node, 116. 

Nodules, 245. 

Nucellus, 274. 

Nucleolus, 41. 

Nucleo-protein, 105. 

Nucleus, 41. 

Nursery plants, 260. 

Nut, 349. 

Nutrition without chlorophyll, 82. 



INDEX 



397 



Oak, quartered, 126. 

Oats, temperature efifect on growth, 
158. 

(Edema, 234, 368. 

Offsets, 188, 189. 

Oil, Castor, 104; Cottonseed, 104; 
Linseed, 104 ; Olive, 104. 

Orchid, pollination, 293 ; seeds, 
372. 

Osmoscope, 227, 228. 

Osmosis, danger, 234 ; described, 
227, 232; explanation, 230; uses 
in plants, 233. 

Osmotic, phenomena, common, 235 ; 
pressure in growth, 233 ; pres- 
sures, 229 ; processes, described, 
232. 

Outgrowths from petals, 334. 

Ovarian wall, 351. 

Ovary, of Buckeye, 350 ; compart- 
ments, 349 ; compound, 273 ; 
described, 274 ; inferior, 275 ; 
simple, 273 ; superior, 275 ; union 
of carpels, 323. 

Ovule, 273, 323; described, 274; 
forms, 272 ; to seed, 354 ; struc- 
ture, 271, 277. 

Oxygen, release by plants, 23. 

Paleobotany, 3. 

Palisade tissue, 30. 

Palm, 60, 127, 136. 

Palmate venation, 66. 

Pandanus, 253. 

Panicle, 337. 

Pansy seed, 376. 

Paper, 205. 

Parasite, 9, 11, 83 ; damage, 85. 

Parasitic Fungi, 367. 

Parrish, 18. 

Parthenocarpy, 354. 

Parthenogenesis, 302. 

Pasteur, 39. 

Pathology, 4, 367. 

Peach Yellows, 369. 

Pearson Fern, 197. 

Peat, 241. 

Pectin, 103. 

Pedicel, 193. 

Peduncle, 193. 

Peg, of Pumpkin, 382. 

Pepsin, 110. 

Peptone, 105. 



Perennials, 114; herbaceous, 114; 
woody, 114. 

Perfumes, 108. 

Perianth, 333. 

Periblem, 264. 

Pericycle, 265. 

Permeable membrane, 228, 235. 

Petal, 270 ; outgrowths, 334. 

Petiole, 16. 

Phanerogams, 12. 

Pharmacology, 4. 

Phenotypically, 310. 

Phloem, 122, 130, 222, 262. 

Phosphates, 242. 

Photosynthesis, 262 ; amount, 25 ; 
definition, 21 ; vs. respiration, 169. 

Photosynthetic equation, 23. 

Photosynthometer, 24. 

Phototropic response, nature, 56. 

Phototropism, 54, 296 ; in Fuchsia, 
55. 

Phyllodia, 80, 81. 

Phyllomania, 203. 

Phyllotaxy, 328; described, 139; 
origin, 143. 

Physiological disturbances, 368. 

Physiology, 3, 5. 

Phytopathology, 4„367. 

Pigments, 108. 

Pine, cross section, 147 ; radial sec- 
tion, 148 ; stem, 126 ; tangential 
section, 149. 

Pineapple, fasciated, 198. 

Pinnate venation, 66. 

Pistil, 273 ; generahzed, 278. 

Pistillate flower, 276, 285. 

Pitcher Plant, 75, 76, 203. 

Pitchers, 9, 74. 

Pith, 116, 132, 265. 

Placenta, 275, 323, 347, 351; dia- 
gram, 324. 

Plant, adult, 9 ; breeding, 4, 371 ; 
breeding, methods, 317 ; definition, 
7 ; diversity, 5 ; food, use of term, 
28 ; foods, 242 ; geography, 4 ; 
Industry, 4 ; insect catching, 87 ; 
primary parts, 7 ; skeleton, 98 ; 
spraying, 97 ; transplanting, 97. 

Plants, numbers, 1. 

Plastid, 41. 

Platy cerium, 186. 

Plerome, 264. 

Plowing, 260. 



398 



INDEX 



Plume, 358. 

Plumule, 374. 

Pollen, 272; germination, 275, 276; 
grains, 286 ; injured by water, 
295. 

Pollination, 277, 370. 

Polyadelphous, 272. 

Polycotyledony, 377. 

Polyembryony, 302, 377. 

Polypetalous, 271. 

Polysepalous, 270, 330. 

Poppy, 348. 

Potentialities, utilization, 206. 

Preservation of sports, 318, 319. 

Pressure gauge on root, 226. 

Procambium, 265. 

Progeotropic, 247. 

Projection of seeds, 364. 

Proliferations, 201, 367. 

Proliferous Pear, 201 ; Rose, 202. 

Propulsion of water, 148. 

Protection, 261 ; of roots, 232. 

Protein, 104 ; composition, 27 ; as 
food for man, 106 ; grains, 105 ; 
kinds, . 105 ; layers, 105 ; where 
made, 27. 

Proteose, 105. 

Prothallus, 306. • 

Protoplasm, 30 ; alveolar structure, 
38 ; appearance, 35, 36 ; char- 
acteristics, 35 ; chemical compo- 
sition, 38 ; composition, 106 
constitution, 37 ; continuity, 40 
definition, 8 ; organization, 40 
properties, 39 ; streaming, 37 
texture, 36. 

Protozoa, in soil fertility, 246 ; in 
soils, 244. 

Pruning, 122, 370 ; uses, 206. 

Pteridophytes, 11. 

Ptomaines, 109. 

Puffball, 87. 



Quinine, 109. 

Raceme, 336 ; determinate, 

indeterminate, 336. 
Rafflesia, 84, 86 ; 268. 
Rainbow Corn, 90. 
Raphe, 274, 376. 
Rattan Palm, 113, 184. 
Receptacle, 193, 271, 275, 351. 
Recessive, 311. 



336 



Reduction division, 285. 

Redwoods, 113, 115. 

Reflex action, 55. 

Regulation, 39. 

Relative transpiration, 47. 

Reproduction, 265 ; asexual, 298 ; 
in Fern, 306 ; sexual, 389. 

Resin, 108. 

Respiration, 111, 112, 266; amount, 
164 ; described, 162 ; in roots, 231 ; 
vs. combustion, 165. 

Respiratory ratio, 162 ; equation, 
165. 

Respiroscope, 162, 163. 

Resting period, 341, 377, 378; na- 
ture, 379. 

Reversions, 201. 

Rhizoid, 215, 250. 

Rhizome, of Sedge, 187. 

Rock, puh^erized, 238. 

Rockweed, 189, 304 ; sex cells, 304. 

Root, aeration, 258 ; aerial, 253, 254, 
256, 257; anatomy, 220, 222; 
anchorage function, 250 ; cap, 217, 
221 ; crops, 258 ; cross section, 
215 ; distinctive features, 212 ; 
distorted, 257 ; in drains, 248 ; du- 
ration, 214 ; economics, 257 ; ex- 
cretions, 243 ; as n foliage, 254, 
255 ; function, 7 ; growing point, 
217, 221 ; growth zone, 218, 221 ; 
hair, 218, 224; hair in soil, 
240 ; hair zone, 218, 221 ; hairs, 
use, 225; length, 250; longi- 
tudinal section, 219 ; need for 
air, 258 ; of Orchids, 254 ; origin, 
223 ; plan of, 225 ; pressure, 
226; protection of, 232; prun- 
ing, 208 ; selective power, 231 ; 
self-adjustments, 247; shortening, 
257 ; special functions, 250 ; as 
spines, 256 ; as storage organs, 
251 ; strains, 216 ; structure, 
215 ; system, typical, 213 ; tip, 
213, 217 ; tip, diameter, 220 ; tip, 
of Radish, 216. 

Rootstock, 187, 188. 

Rose, green, 201, 341 ; of Jericho, 
359, 360. 

Rotation of crops, 260. 

Rots, 367. 

Rubber, 108. 

Rubus squarrosus, 193. 



INDEX 



399 



Russian Thistle, 359. 
Rust, 367 ; of Wheat, 90. 

Saccharose, 100. 

Sachs, Lectures, 36. 

Sand-box, 365. 

Sap, rise in trees, 147 ; theory of 
ascent, 149 ; wood, 124, 145. 

Sapling, 387. 

Saprophyte, 11, 83. 

Sargent, Plants, 117. 

Sarracenia, 75, 76, 246. 

Scabs, 367. 

Scape, 193. 

Science, aim, 12 ; applications, 5. 

Scion, 208. 

Sclereids, 265. 

Sclerenchyma, 130, 265. 

Scott, Botany, 116. 

Seasonal cycle, 388. 

Seaweeds, 12. 

Secretion, 266. 

Secretions, 107. 

Seed, 390; albuminous, 374, 375; 
characteristics, 372; coat, 351, 
375; condition of life, 378; du- 
ration of life, 377, 380 ; economics, 
385 ; ex-albuminous, 374, 376 ; 
function, 8 ; plants, 10 ; pro- 
jection, 364 ; structure, 373. 

Seedhng, 9, 387 ; of Radish, 218. 

Selection of variations, 318. 

Self-adjustments, 55, 266. 

Semi-permeable membrane, 228, 235. 

Sempervivum, 188. 

Sepals, 269. 

Sex, cells, fusion, 278, 279 ; estab- 
lished, 304 ; origin, 302 ; origin, 
summary, 308 ; in plants, 307 ; 
stages in development, 303. 

Sexual organs, 307. 

Shade, growth under, 95. 

Shoot, 190. 

Shrubs, 9. 

Side roots, origin, 223. 

Sieve, plate, 152; tube, 31, 131, 152, 
262. 

SiKcles, 348. 

Skeleton of plants, 98. 

Skunk Cabbage, 268. 

Sleep movements, 57, 61. 

Slime-mold, 39, 357. 

Slips, 259. 



Smuts, 367. 

Snapdragon, 274. 

Soil, composition, 237 ; cultivation, 
260 ; solution, 241 ; structure, 
238, 239. 

Solomon's Seal, 191. 

Sorus, 326. 

Spadix, 338. 

Spathe, 338. 

Special creation, 315. 

Species, 7. 

Spectroscope, 53. 

Sperm, 277 ; cell, 9. 

Spermatophytes, 10. 

Spermatozoid, 280, 304. 

Sphagnum, 115. 

Spike, 335, 337. 

Spines, 192, 193, 256 ; Barberry, 81 ; 
Echino cactus, 80 ; morphology, 
79 ; significance, 79. 

Spongy tissue, 30. 

Spontaneous generation, 39, 40. 

Sporangium, 324, 325. 

Spore, 324, 373 ; asexual, 301 ; cases 
of Mold, 302 ; dissemination, 360. 

Sporophore, 84. 

Sporophyll, 325. 

Sports, 205, 319; preservation, 318, 
319 ; seed, 319. 

Spot diseases, 367. 

Spraying, 371. 

Squirting Cucumber, 365. 

Stamen, 272 ; irritable, 297. 

Staminate flowers, 276, 285. 

Starch, as food for man, 103 ; for- 
mation under light screen, 21 ; 
formation vs. osmosis, 234; for- 
mation in presence of CO2, 22 ; 
grains, 101 ; grains, typical forms, 
102 ; kinds, 101 ; sheath, 130. 

Stele, 264. 

Stem, anatomy, 128, 129, 131; 
characteristics, 113 ; columnar, 
179 ; as conducting mechanism, 
150 ; creeping, 187 ; deliquescent, 
181 ; economics, 205 ; endogenous, 
127 ; excurrent, 179, 180 ; exog- 
enous, 127 ; function, 7, 53 ; general- 
ized diagram, 125 ; herbaceous, 
115 ; special function, 191 ; stor- 
age, 191; structure, 115; sym- 
metry, 181, 182; tissues, 116; 
tissues, generalized, 117 ; tissues, 



400 



INDEX 



herbaceous stem, 116 ; trailing, 
187 ; traveling, 188 ; typical leaf- 
bearing, 115 ; various forms, 179 ; 
woody, 119. 

Stevens, 119. 

Stigma, 273. 

Stimulus, 54 ; perception, 178. 

Stipule, 16 ; morphology, 80 ; special 
forms, 82. 

Stock, 208. 

Stolon, 188, 189. 

Stoma, 19, 49, 262; clogging, 96; 
diagram of number and area of 
opening, 51 ; number, 50 ; position, 
50. 

Storage, 266 ; battery, 167. 

Strasburger, Textbook, 40. 

Streaming of protoplasm, 37. 

Strophiole, 374. 

Structural features, 13. 

Strychnine, 109. 

Style, 273. 

Suberin, 98. 

Subsoil plowing, 260. 

Substitutions, 201. 

Sucker, 214. 

Sucrose, 100. 

Sugar, 100 ; cultivation, 101 ; kinds, 
100; Maple, 181. 

Sun scalds, 368. 

Sundew, 246. 

Sunflower head, 143. 

Support of foliage, 265. 

Suspensor, 354. 

Symbiosis, 244. 

Systematic botany, 2. 

Tceniophyllum, 255. 

Tannin, 110. 

Tap root, 212, 214, 250. 

Taxonomy, 2. 

Telegraph Plant, 81, 83. 

Tendrils, 9, 76, 192 ; mode of oper- 
ation, 79. 

Thallophytes, 12. 

Thallus, 190. 

Thein, 109. 

Theobromine, 109. 

Thigmotropism, 77. 

Thyrsus, 337. 

Tissue systems, diagram, 263. 

Tissues, definition, 8 ; healing, 122 ; 
summary, 261. 



Topiary work, 207. 

Torsions, 201. 

Toxin, 173. 

Tracheae, 146. 

Tracheid, 31, 146. 

Traction, 149. 

Transfer, of water and food, 144. 

Translocation, of food, 151* 

Transmission of acquired characters, 

315. 
Transpiration, 43, 262 ; amount, 43 ; 

constants, 44 ; demonstration, 43, 

44 ; effect of external conditions, 

47 ; effects, 96 ; fluctuations, 45 ; 

plant prepared for study, 45 ; 

record, 47 ; reduction, 69 ; role, 

49; significance, 51. 
Transpirograph, 45, 46. 
Traumatropism, 249. 
Tree, 9 ; crotch, supported, 211 ; 

height, 150 ; lawn, 181 ; surgery, 

211. 
Tree Fern, 61. 
Trichomes, 19, 70. 
Tropical undergrowth, 59. 
Truth, test for, 13. 
Tuber, 9, 192. 
Tubercles, 245. 
Tuberous roots, 251. 
Tulip Tree, cross section, 121. 
Tumbleweed, 359. 
Tumboa, 71. 
Tumors, 200. 
Twin fruit, 196, 199, 367. 
Twiners, 185. 

Umbel, 337. 

Unit character, 309. 

Vallisneria, 284, 287. 

Valvate, 329. 

Variability, 39. 

Variation, 308; defined, 308, 314; 

selection, 318. 
Vascular bundles, 116, 264. 
Vegetables, 206 ; vs. fruits, 346. 
Vegetative, bodies, specialized, 299 ; 

parts, potential, 300. 
Veinlet, 31. 
Veins, 29, 118; of leaf, 17; netted, 

17; parallel, 17; use, 144. 
Venation, 65 ; netted, 65 ; palmate, 

66 ; parallel, 66 ; pinnate, 66. 



INDEX 



401 



Ventilation, 369, 371. 

Venus Fly-trap, 76, 77, 246. 

Vernation, 137. 

Viability, 380 ; tests, 386. 

Violet, seed pods, 364. 

Vitalistic conception of nature, 39, 40. 

Vitality, suspension, 377. 

Warming, 288. 

Water, capillary, in soils, 239 ; 

flotage by, 360 ; hydrostatic, 238 ; 

hygroscopic films, 239 ; in seeds, 

377 ; in soils, 238 ; uses in plants, 

224. 
Water culture, 243 ; described, 242. 
Water Lily seed, 361. 
Water plants, 9, 61, 251 ; roots, 

252. 
Wax, 111. 

Weeping Birch, 182. 
Welwitschia, 71. 
Whorl, 140. 
Wiesner, 291. 
Wild Geranium, 334. 



Wilting, 48. 

Wilts, 367. 

Wind, effects on trees, 183 ; pol- 
linated flowers, characteristics, 288 ; 
pollination, 288 ; waftage, 358. 

Windburn, 48, 97, 259. 

Wing, 358 ; on fruits, 350, 351. 

Winter-killing, 259. 

Witches' brooms, 198, 199. 

Wood, 130 ; fibers, 265 ; grain, 124 ; 
parenchyma, 130. 

Wooden Flower, 200. 

Xanthophyll, 89, 90, 91 ; composi- 
tion, 108. 
Xenia, 353. 
Xerophyte, 190. 
Xylem, 122, 130, 222, 262. 

Yeast, 169. 

Yucca, pollination, 293. 

Zoospores, 301, 357. 
Zymase, 110. 



/^ 



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different phases of Botany, the characteristics of good 
botanical teaching, scientific drawing and exposition, the 
planning and equipment of laboratories, the preparation 
of museum and other collections, botanical books, with 
bibliography, and current errors which should be avoided. 

The second part contains suggested outlines, with full 
practical directions for a general or introductory course in 
accordance with the results of the best recent experience. 
Detailed information is given concerning physiological 
experimenting. In an Appendix are reprinted the two 
syllabi prepared by the Botanical Society of America, 
and the Association of Colleges and Secondary Schools of 
the North Central States. The book is thus concerned 
directly and practically with the problems which face the 
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the United States and provides adequate keys and descrip- 
tions for their identification. Technical description of 
each division, order, family, genus, and species is given. 
Many parasites not yet known in the United States are 
briefly mentioned, especially those of greater importance, 
or those which are likely to invade America. Non-para- 
sitic groups closely related to those which are parasitic 
have been included in the keys in order to give the student 
a larger perspective. At least one illustration of each- 
genus which is of importance in the United States has 
been included. Abundant citations to the more important 
papers are given, so as to put the student in touch with 
the hterature of the subject. This is, however, the only 
work in English covering this ground, and will, therefore, 
be welcome to students who have previously been obliged 
to rely solely upon very expensive treatises in Latin or 
German, and upon numerous monographs, magazine arti- 
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Students of Plant Diseases are naturally divided into two categories. 
First : Those who wish to recognize and treat diseases, without the bur- 
den of long study as to their causes ; Second : Those who desire to 
study the etiology of diseases, and to become familiar with the parasites 
which are often their cause. The present book is designed to meet the 
needs of the first of these two classes of readers, and particularly for 
such students in the Agricultural Colleges and Agricultural High 
Schools. It indicates the chief characteristics of the most destructive 
plant diseases of the United States caused by cryptogamic parasites, 
fungi, bacteria, and slime moulds, in such a way that reliable diagnoses 
may be made, and fully discusses the best methods of prevention or cure 
for these diseases. In this volume only such characters are used as 
appear to the naked eye or through the aid of a hand lens, and all tech- 
nical discussion is avoided in so far as is possible. No consideration 
is given to the causal organism, except as it is conspicuous enough to 
be of service in diagnosis, or exhibits peculiarities, knowledge of which 
may be of use in prophylaxis. While, in the main, non-parasitic dis- 
eases are not discussed, a few of the most conspicuous of this class are 
briefly mentioned, as are also diseases caused by the most common 
parasitic flowering plants. A brief statement regarding the nature of 
bacteria and fungi and the most fundamental facts of Plant Physiology 
are given in the appendix. Nearly two hundred excellent illustrations 
greatly increase the practical value of the book. 



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new branch of science little attention is given, in the 
first flush of investigation, to the logical concepts and 
philosophical principles which underlie it. This lack 
of philosophical poise is now becoming rather generally 
apparent in genetic research. The present book is a 
contribution to the methodology of genetics, in a philo- 
sophical sense. It attempts first to examine carefully and 
then to appraise the value of the more important current 
methods of attacking the problems of heredity and breed- 
ing, including the statistical or biometrical method, Men- 
delism, etc. The book should, on the one hand, interest 
every professional student of biology in any of its 
branches, who is at all concerned with the question of the 
philosophical foundation of his science. On the other 
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lay the foundation for a knowledge of invertebrate anat- 
omy. It is intended for use in the course in Invertebrate 
Zoology which is preceded by the course in General 
Biology or Elementary Zoology. 

The treatment of the subject differs somewhat from 
the usual. Each chapter consists of two parts — a mono- 
graph in which a description is given of the animal 
selected as representative of its class, and instructions 
for the student to follow in dissection. The descriptions, 
while short, are sufficiently detailed to include obvious 
structures of specific value. The monographs are based 
partly on work done by others, partly on the author's 
own dissections and investigations. 

The species used are almost all American, and, with 
the exception of the earthworm, are entirely different from 
those used in the General Biology course. 



THE MACMILLAN COMPANY 

Publishers 64-66 Fifth Avenue New York 



Morphology of Invertebrate Types 



By ALEXANDER PETRUNKEVITCH, Ph.D. 

Assistant Professor of Zoology in the Sheffield Scientific School of Yale 
University 



A laboratory guide which will enable the student to 
lay the foundation for a knowledge of invertebrate anat- 
omy. It is intended for use in the course in Invertebrate 
Zoology which is preceded by the course in General 
Biology or Elementary Zoology. 

The treatment of the subject differs somewhat from 
the usual. Each chapter consists of two parts — a mono- 
graph in which a description is given of the animal 
selected as representative of its class, and instructions 
for the student to follow in dissection. The descriptions, 
while short, are sufficiently detailed to include obvious 
structures of specific value. The monographs are based 
partly on work done by others, partly on the author's 
own dissections and investigations. 

The species used are almost all American, and, with 
the exception of the earthworm, are entirely different from 
those used in the General Biology course. 



THE MACMILLAN COMPANY 

Publishers 64-66 Fifth Avenue New York 



J 



J. 



